From the support page:
On December 1, 2018, the Groove Music iOS and Android apps will be retired. Your personal music files will continue to be available in OneDrive. You can keep listening to those files, and your playlists, with the Windows 10 Groove Music apps on PC, Xbox, or Windows Phone.
What's happening to the Groove Music iOS and Android apps?
The Groove Music iOS and Android apps will be retired on December 1, 2018. After that date, the apps won't work and should be uninstalled.
What will happen to my music?
Your personal music files will still be available on OneDrive. You can access OneDrive online or through the OneDrive app on any supported device. To keep playing your music and playlists in Groove, use the Windows 10 Groove Music apps on PC, Xbox, or Windows Phone.
The intersection of Groove Music application users on iOS and Android who also have large music collections stored in OneDrive must be fairly small. I guess there could be ex-Windows 10 Mobile users who moved to another platform and simply retained their online collection, but surely the fix there is just to download everything from OneDrive and either use locally within the new phone or even re-upload to Google Play Music (which is free and unlimited for tracks you own)?
In short, this isn't really a major story. But I wanted to post something to reassure Windows 10 Mobile users that Groove is the native music player on the platform and that it's not going away - by definition. Mind you, it's also not liable to see any updates or new features, so it's something of a hollow congratulation.