restic does append-only backups. This means when you call backup no existing file/object in the repository is removed or changed. Only new files/objects are generated. This makes it perfectly work in combination with something like object lock or --append-only mode of rest-server.
The only thing you have to take care about: restic uses locks which do need a deletion after the operation is done. If you can configure your object lock on subpaths, exclude /locks from it and you are fine. If you can’t do this, take a look at the lock-free alternative rustic.