So, I totally remember this pop-up and luckily until now it has not directly impacted me. I auto-backup my photos/videos off my phone and do move non photos/videos on/off nextcloud on occasion but haven't noticed I could no longer upload regular files.
Luckily...
I self-host Nextcloud I already use F-Droid
So, it wasn't much work to uninstall the play version and instlall the f-droid version. I should probably go through all my apps and see what's available on f-droid and swap. But 1) that's a lot of work and 2) there are some features that break when you do that. I know some mapping software won't work on Android Auto if not from the play store.