Currently, you can either provide the `BORG_REPO_PERMISSIONS` env var to borg [0] or `--permissions` flag to `borg serve` [1]. You can then enforce this as part of your `authorized_keys` command, for example.
[0] https://github.com/borgbackup/borg/blob/3cf8d7cf2f36246ded75...
[1] https://github.com/borgbackup/borg/blob/3cf8d7cf2f36246ded75...