Yes, we're in an odd phase of the OS, where most of it 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.
Note that some dates are approximate, since even we at AAWP can't see into the future with certainty!
|Security updates||Until December 2019 (for newest 1709 branch). Theoretically, vulnerable after this, though in practice Windows 10 Mobile will be battle hardened by then and also a miniscule target for attackers, so the lack of update isn't actually an issue|
|Auto-backups||Until March 2020 - it's not clear whether manual backups will be possible after this|
|Restoring from auto-backups||Until March 2021|
|Store||Application updates and installs will be fine until well into 2020 and possibly 2021|
|Auto-Photos upload to OneDrive||Until December 2020 - after that you'd have to upload images manually|
|OneDrive music streaming||Stopped from May 2019 onwards, at least from within Groove Music app. You can still stream or download from third party applications.|
|OneDrive||No cessation date announced, likely 2022 or beyond|
|Outlook Mail, Calendar
||No cessation date announced, likely 2022 or beyond|
|Office UWP applications||No cessation date announced, likely 2022 or beyond|
|Maps/Navigation||No cessation date announced, likely 2022 or beyond|
|News, Weather||No cessation date announced, likely 2022 or beyond|
|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.|
|Skype||No cessation date announced, but it's 100% online and requires server integration, obviously, so I wouldn't be surprised if a date was announed later in 2019. Maybe March 2020?|
|Official client is a PWA, but works well apart from lack of push notifications. API limitations at Twitter's end mean that there's no fully working third party alternative|
|Official (Osmeta-based, derived from iOS codebase) client is being withdrawn at the end of April 2019, though it's not clear whether installed copied will stop working (I guess not initially). 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|Facebook Messenger||Official (Osmeta-based, derived from iOS codebase) client is being withdrawn at the end of April 2019. The solutions linked in the row above also include basic Messenger support, though. Apparently. Never used the system in my life!|
|Official (Osmeta-based, derived from iOS codebase) client is being withdrawn at the end of April 2019. There are several third party alternatives, my favourite of which is Winsta UWP.
|The official WP8.1 client continues to work really well. No end date for support has been announced, I suspect it will be (at least) December 2019, as per Windows 10 Mobile itself.|
|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.|
|Viber||This messaging app has a first party UWP application, but reviews have been patchy - it's not actively updated. Ping me if you have more information - I've never used it!|
|Snapchat||This has never been on Windows phones and never will be. Still...|
|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|
|Spotify||The official WP8.1 application still works, but is buggy as heck under the latest Windows 10 Mobile. I'd recommend the 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!|
|Netflix||The official WP8.1 application still works well, albeit 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.|
|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.|
|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. 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 completely 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 welcome!
PS. Another valid question might be what will happen to AAWP itself in 2020 and beyond? Rafe and I will chat about resources, of course, but a) do please join the occasional guest writers if you have something to say/review/discuss, email me at firstname.lastname@example.org, and b) well, you know where my tip jar is...(!)