> Except if you keep overwriting a flash-based storage system, at some point that flash storage gets destroyed (wear level). You can absolutely achieve such by having a near full filesystem on flash. Mechanical harddrives or partitions don't suffer from this issue.

Wearing out is yet a different thing. I've had this happen on an SD card. It would refuse to write anything new, although it reported being mostly free. But the stuff that was already on it was readable.

I've had SD cards that got full. They didn't lose any data, and once I'd moved the things off them, they became usable again.

Granted, this was with a digital camera, so using fat32 at the time, so no fancy FS.

Without more information on the exact mount error and btrfsck --readonly output, it's a guessing game.

In the ordinary cases, btrfs full behavior is the same as other filesystems. It gets full, you can delete files. Keep in mind deleting files on any cow filesystem is a write that consumes free space before space is freed from the delete operation. There is reserve space for this. If you hit an edge case (which would be a bug) there's currently no known data eating bugs. But it's not always obvious to the user their data hasn't been eaten if the filesystem won't mount. As in, this is indistinguishable from data loss. Nevertheless it's a serious bug so if you have a reproducer it needs to be reported.