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.