It would be very useful to have the option -one-file-system, which shall prevent the backup process from copying files and folders from another file system. It’s a quite popular option among other tools
What other tools is this popular in? How is the utility supposed to tell which filesystem to include files from?
E.g. Borgbackup, rsync or tar. The filesystem where the root of the repository is used and if a subdirectory is in another FS, then that boundary is not crossed. An exception would be, that a subdir would explicitly be specified to be backed up.