Yes, we're in an odd phase of the OS, where most of the stuff that ever works still works (and is supported), but things are falling off the edges, as it were. Which means that it's hard to keep track of it all - hence the table below, which I'll keep updated every so often. [last update 26th Oct 2019]
Note that some dates are approximate, since even we at AAWP can't see into the future with certainty! I've colour coded table cells, with pale red for 'on the way out, with workarounds and caveats' and red for 'no hope!'...
|Security updates||Until December 2019 (for newest 1709 branch only, now - here's the November update). Theoretically, vulnerable after this, though in practice Windows 10 Mobile is now battle hardened and also a miniscule target for attackers, so the lack of updates won't actually be an issue.|
|Web browsing (Edge)||Edge remains a competent browser for most sites, plus it syncs your history and favourites in the usual way. But its development stopped as of Windows 10 branch 1703 over two years ago, effectively, since the 1709 branch's Edge wasn't fully rounded out (in terms of service worker and other background operation). Which makes it something of a dead end if you're looking for more. Third party browsers like Monument Browser do more, though ultimately have the same underlying restrictions for PWAs and some other HTML5-reliant sites.|
|Auto-backups||Until March 2020 - it's not clear whether manual backups will be possible after this. Watch this space.|
|Restoring from backups||Until March 2021 - after this (i.e. eighteen months from the date of this article), if you hard reset a Windows 10 Mobile phone then you'd have to rebuild it manually, installing from the Store, etc. Which will still hopefully function.|
|Store apps||Application updates and installs should be fine until at least the end of 2021. My best guess for the Windows 10 Mobile Store being switched off is March 2022.|
|Store client||Ignore scare stories about the Store client itself not being updated anymore - updates with bug fixes and security tweaks will continue for years (2021+), albeit with new features and UI fanciments only for Redstone 3 and above (i.e. on Desktop/Hololens, etc.) And yes, some of these updates will have knock on effects for Windows 10 Mobile - anyone else had problems with the Store needing a manual 'Refresh' recently?|
|Auto-Photos upload to OneDrive||Until December 2020 - after that you'd have to upload images manually. It's worth noting that the OneDrive client on iOS and Android is excellent - if you do switch at some point then you essentially carry on with the same interface and auto photo stream in the cloud (subject to your storage plan, e.g. 1TB with Office 365)|
|OneDrive music streaming||Stopped from May 2019 onwards, at least from within Groove Music app. You can still stream from OneDrive or download from both first and third party applications - see the linked article for suggestions.|
|OneDrive||No cessation date announced, likely 2022 or beyond.|
|Outlook Mail, Calendar
||No cessation date announced, likely 2022 or beyond. Outlook app development is all on newer Windows 10 branches though, so the current one on Windows 10 Mobile is only receiving small bug fixes and none of the sexier stuff you'll read about in the news.|
|Office UWP applications||No cessation date announced, likely 2022 or beyond. Again, app development is all on newer Windows 10 branches though, so the current Office UWP apps on Windows 10 Mobile are only receiving bug fixes and no new features.|
|Maps/Navigation||No cessation date announced, likely 2022 or beyond. 'Automatically update maps' doesn't seem to work anymore, but you can still check for updates and pick up new maps manually, in 'Settings/Apps/Offline maps'. Worth doing every couple of months, I reckon, there's usually a new map update when I check on each device!|
|News, Weather||No cessation date announced, likely 2022 or beyond. Functionality for the UWP apps under Windows 10 Mobile has been frozen for a while, but News in particular is still getting tweaks to encompass new news sources and options.|
|Microsoft To-Do||No cessation date announced, but application updates have now stopped on Mobile, with new features only appearing on higher Windows 10 branch numbers, for the Desktop. Compatibility with base to-do data is likely until 2022 though.|
|Cortana||This should carry on working into 2020, though the withdrawal of Cortana on iOS and Android does lead me to suspect that Cortana results will break at some point in the year.|
|Films & TV||(aka 'Movies & TV') No cessation date announced, but several rumours. Clever money has new purchases stopping some time in 2019 and no access to past purchases stopping in 2020. Just a guess. I've checked and past purchases are still available, the DRM still works, and with no warning messages in-app. Watch this space.|
|Skype||No cessation date announced for the UWP app (the Desktop is now back to Win32, effectively), but it's 100% online and requires server integration, obviously, so I wouldn't be surprised if a date (early 2020?) for Skype on Windows 10 Mobile was announced by the end of 2019, tying in with the end of support for the OS itself.|
|Official client is a PWA (link is to latest update), but works very well apart from lack of push notifications. Oddly, the Twitter team has stopped publishing changelogs to the PWA, but I'll keep an eye on this. API limitations at Twitter's end mean that there's no fully working third party alternative (at least, one that also has access to DMs).|
|Official (Osmeta-based, derived from iOS codebase) client has now been withdrawn from the Store, as of June 2019, and even if you have it previously installed then it crashes and burns. There are numerous Facebook 'scraping' applications and options though, and these run faster and more efficiently than the Facebook original client (go figure!)|
|Facebook Messenger||Official (Osmeta-based, derived from iOS codebase) client also now not available anymore. Good riddance, arguably, since it was bloated. The solutions linked in the row above also include basic Messenger support, though. Apparently. Never used Messenger in my life!|
|Official (Osmeta-based, derived from iOS codebase) client has now been withdrawn, being part of the Facebook 'empire'. There are several third party alternatives though, the clear favourite of which is Winsta UWP.
|The official WP8.1 client continues to work well, though it has been announced that it will stop working after December 2019, as per Windows 10 Mobile official support timescale. It's also going to be withdrawn from the Store some time after June 2019, i.e. to stop new downloads. At the time of writing it's still in the Store, so if you're going to do a reset and rebuild of your phone then do it sooner rather than later if you depend on this service, at least for two more months.|
|Telegram||Plenty of options here, with an official 8.1 application that is gradually falling into disrepair, but also an official PWA that works well (though with no push notifications) and an actively developed third party app, Unigram UWP. For messaging anyway, voice and video calls aren't supported.|
|Viber||This messaging app has a first party UWP application, but reviews have been patchy - it's not actively updated. According to readers it still works fine for messaging, voice and video though.|
|Snapchat||This has never been on Windows phones and never will be. Apparently the founder hated Microsoft and Windows....|
|There's no official client for this, but pinterest.com in Edge seems to be a PWA and acts like an application. Plus, if you still want a native UWP application experience, there's 'Piny - Pinterest with love' UWP|
The official WP8.1 application still technically runs, but has now lost search functions and is clearly on the way out. If you're serious about Spotify though, then you'll have the £10/month premium tier, and then you can use the new and rather swish Spotimo UWP.
There's also the quirky but rather wonderful Spoticast Music UWP, though it cheats in terms of its music sources, which can be confusing. Still, it's gloriously bonkers and very pretty! Or try the Spotify Web Wrapper UWP, which presents a front end to the Desktop Spotify web experience and which seems to work very well, though the developer is somewhat at the mercy of what Spotify might do in the future. Still, options!
|Netflix||The official WP8.1 application still works, albeit being slow to load video streams. It hasn't been updated for years though and (again) I wouldn't be surprised to see Netflix stop supporting it after Dec 2019, tying in with deadlines on this page generally.|
|Amazon Prime Video||Amazon has never had an official application for Windows phones, but if you're happy to fiddle a little in Edge then you can watch this on the go. It's not trivial though and is a complete pain in the proverbial compared to the slick experience in a dedicated app on other platforms.|
|Deezer||There's a rather good official UWP client for this, so no need to look further. I'd expect this to work well into 2020, and possibly beyond.|
|YouTube||One of the more famous spats between Google and Microsoft, this hasn't had an official application for years, though of course it works fine in the Edge browser. I'd recommend myTube!, Perfect Tube or Awesome Tube though, all UWP applications with slick interfaces and all kept bang up to date.|
|BBC iPlayer||This has been browser-only for years under Windows phone and Windows 10 Mobile, but it's fine. There's the usual 'log into the BBC servers' and you're in and watching. There's no downloading of programmes for watching later though, for DRM reasons.|
Do please let me know, by email or in the comments, if there are any inaccuracies in the above table or if you have information to update it. Thanks.
Delivering a 'report card' for the status above is tricky. I'd say that the OS is viable still, with the obvious omissions (tap to pay, IoT support, online banking apps) that have been there for a while now. There are certainly enough workarounds and alternative applications for most people.
But your comments and updates welcome! This is an update to the original story, so I've left previous comments in place.
PS. Another valid question might be what will happen to AAWP itself in 2020 and beyond? Rafe and I are chatting about resources, of course, but a) do please join the occasional guest writers if you have something to say/review/discuss, email me at email@example.com, and b) well, you know where my tip jar is...(!)