The 8.1 Skype application should carry on working for a few months yet, it's certainly fully functional on my own 8.1 devices as of today. I'd estimate that things will break in the Spring when Redstone 2 hits in the Windows 10 Mobile world and when all sorts of things get reconfigured server-side.
There's more though. The new Skype UWP client claims to just need 'Windows 10 Mobile' when it actually needs the 'Anniversary Update' onwards (i.e. Windows version 1607). This isn't an issue for most users because fully eligible production W10M phones will have already received their update from '1511' to '1607', i.e. from 'Threshold' to 'Redstone'/Anniversary Update, and so will be fully compatible with the Skype UWP.
However, there are users who, like me, have a number of older non-W10M-eligible smartphones (think Lumia 920) which were upgraded to W10M through the Insider programme and are now 'stuck' at Windows 10 Mobile Threshold. If such a user had already installed the Skype UWP then it should carry on working and be updated, but if (again, like me on one of my test phones) they hadn't got round to it a few months ago and still only had the 8.1 client then they're stuck with this. The Store client shows the new UWP app, but only offers 'launch', which then dutifully launches the old client. Akin to 'falling down the cracks', I think!*
* I have a hunch that if I hard reset my ex-Insider W10M Threshold phone, it'll successfully pull in the Skype UWP from the restore link set - a job for a rainy day, I think(!) Anyone else in this boat?
There's another class of user which will be affected, those with official Windows 10 Mobile handsets from niche manufacturers who haven't been keeping things up to date in terms of liasing with Microsoft. MSPU reports:
...there are many Windows phones that aren’t supported for an upgrade past version 1511 – including some which shipped with Windows 10 Mobile oddly enough. The Moly X1 for one, despite having internal specs to rival the Lumia 830 or 640, remains stranded on Windows 10 Mobile 1511 without access to either the Windows Insider program or any other means of update. I have no doubt that the case is the same for other lesser- known Windows devices.
Mind you, as I said recently, of the abandonment of the 8.1 client:
Commentators online have tried to point out that this move 'leaves behind' 80% of the Windows mobile userbase, but this is twisting the stats. If you look at Windows mobile users who actually USE Skype (i.e. the enthusiasts, the tech-literate) then the majority of these are now on phones from within the last three years, meaning that they can upgrade all the way to Redstone and use the Skype UWP app....
Which is about as expected. And returning to my theme above, anyone with a 2012/2013 Windows Phone 8.1-only phone (i.e. can't be upgraded to Windows 10 Mobile and UWP compatibility) can't really complain that one of the Internet services on the phone stops working 4-5 years later. In the mobile world, that's an eternity. And 99% of those Windows mobile users who use Skype day to day will definitely be on a UWP-capable device by 'early 2017'.
In short, this is a non-issue, though I'd still like to see drastic performance improvements from the Skype UWP. Come on guys rip out all those debug routines!