Microsoft ended Windows Phone 8.1 support 11th July 2017 and today the company announced the phasing out of support for the Windows Phone 8.1 store.
This does not mean Windows Phone 8.1 apps will disappear – in general, they will still be available on Windows 10 Mobile handsets, but soon those still on Windows Phone 8.1 handsets will no longer have access to them.
In around 2 months, on the 31st October 2018, Microsoft will stop accepting new app submissions with Windows Phone 8.x or earlier or Windows 8/8.1 packages (XAP or APPX).
In a year’s time, on the 1st July 2019, Microsoft will stop updating apps on Windows Phone 8.1. App updates will still be pushed out to Windows 10 devices, even if they are WP8.1 apps.
Finally, on the 1st July 2023, Microsoft will also stop supporting Windows 8 and Windows 8.1 devices. Developers will still be able to publish updates to all apps (including those with Windows 8/8.1 packages). However, these updates will only be made available to Windows 10 devices.
Given the age of Windows Phone 8.1 I suspect very few end users are affected, and most companies would have completed their transition to alternate operating systems long before then.
The full details can be read at Microsoft here.