Microsoft today announced many deadlines for developers which have built Windows 8 and Windows Phone 8 apps. The Microsoft Store will prevent accepting newly discovered apps with Windows Phone 8.x or earlier or Windows 8/8.1 packages (XAP and APPX) on October 31, 2018.

In July 2019, Microsoft will stop distributing app updates to Windows Phone 8.x or earlier, whereas in the July 2023, Microsoft stops distributing app updates to Windows 8/8.1 devices. After that, Microsoft says updates is only made available to customers using Windows 10 devices - although who is familiar with what Windows may be like in 5yrs.

(Don't confuse the Microsoft Store with Microsoft's physical stores. Microsoft rebranded the Windows Store as being the Microsoft Store in October 2017.)

Microsoft has given Windows app developers the accompanying timeline to help them plan:

October 31, 2018 - Microsoft will eradicate accepting new app submissions with Windows Phone 8.x or earlier or Windows 8/8.1 packages (XAP or APPX). This is going to not affect existing apps with packages targeting Windows Phone 8.x or earlier and/or Windows 8/8.1 specialists continue to submit updates just before the next corresponding deadline.
July 1, 2019 - Microsoft does away with distributing app updates to Windows Phone 8.x or earlier devices.
Developers are still able to publish updates to all of apps (including people Windows Phone 8.x or earlier packages), these kind of updates is only made available to Windows 10 devices.
July 1, 2023 - Microsoft will eradicate distributing app updates to Windows 8/8.1 devices. Developers it is able to publish updates everybody apps (including those with Windows 8/8.1 packages), nonetheless, these updates are only made available to Windows 10 devices.

Microsoft is encouraging affected developers to port their apps towards universal Windows platform (UWP). UWP allows developers to produce a single app that changes driven by your tool and screen size. One app could help you on your Windows 10 computer, Windows 10 tablet, Windows 10 Mobile smartphone (it's possible to skip this specific), Xbox One console, and HoloLens headset.