The recently lost file upload feature in the Nextcloud app for Android

This is also why the official SyncThing Android app stopped being distributed. There is a fork but it's not available on the Play Store.

The problem with the SyncThing Android app is that it's just a wrapper around SyncThing, which is a Go library, but SAF does not give you simple file descriptors you can use in native code. Instead, you get "content://" URLs, and you need a Java/Kotlin bridge to convert these to file descriptors. That would need to be done in SyncThing itself (EDIT: or some other trickery, because it seems like syncthing-fork made it work somehow).

However, AFAIK, this problem would not apply to the NextCloud app.

You can get simple file descriptors for SAF, but you do need to ask for them via Java APIs.