From the Microsoft page:
Support for Windows Phone 8.1 operating system ended on July 11, 2017. As a culmination of the end of support process, the Windows Phone 8.1 Store will shut down on December 16, 2019. Lumia phones using Windows Phone 8.1 and any apps that have already been downloaded from the Store may continue to work after this date. However, users may experience a further reduction in functionality as follows:
- Users will not be able to download any additional apps to the phone. Users should make sure to download all applications that they might want before December 16, 2019.
- There may be some degradation in app functionality going forward as updates are no longer available via the Store. Users should check with app publishers if they have questions.
- Reinstallation of apps will no longer be possible. If a user removes an app from the phone it will not be possible for such app to be restored. This includes phone reset as well as use of the Windows Device Recovery Tool (WDRT) where apps are deleted or removed. Users should take this into consideration before choosing to reset the phone or use WDRT on or after December 16, 2019.
Certain Lumia phone models can be upgraded to Windows 10 Mobile. Users who choose to upgrade an eligible device should note that Window 10 Mobile is already out of support or, in some cases, will be by the end of 2019. This means the operating system will no longer be updated (including no additional security updates), though the Store is currently still functional.
The page then lists the Lumias that are eligible for the Windows 10 Mobile upgrade, though I'd have to disagree with some of the lower specced models, on which the experience will be appalling(!)
- Microsoft Lumia 430
- Microsoft Lumia 435
- Microsoft Lumia 532
- Microsoft Lumia 535
- Microsoft Lumia 540
- Nokia Lumia 635 (only 1GB RAM variant)
- Nokia Lumia 636
- Nokia Lumia 638
- Microsoft Lumia 640
- Microsoft Lumia 640 XL
- Nokia Lumia 730
- Nokia Lumia 735
- Nokia Lumia 830
- Nokia Lumia 929
- Nokia Lumia Icon
- Nokia Lumia 930
- Nokia Lumia 1520
Over-the-air upgrade to Windows 10 Mobile requires Upgrade Advisor application that can be downloaded from the Windows Phone 8.1 Store. Users must download Upgrade Advisor before December 16, 2019 if they want to use this option. Upgrades can also be performed with Over-the-Cable (OTC) Updater PC tool if users do not have the Upgrade Advisor app.
All good to know anyway. The only people still using Windows Phone 8.1 that I'm aware of in this community are on the imaging classic Lumia 1020 - it'll be sad to think that this could no longer be fully set-up from scratch in the event of a hard reset, though note that the core system and imaging applications don't depend on the Store, so the phone will carry on functioning for the absolute basics.