I a world of 8 billion people, it's important to realize what one considers a "must" is another person's "don't care". I'm one of them.

You are entitled to your own personal opinion. Backup software must support append only. Otherwise it's a glorified version of rsync(rclone).

restic has supported append-only repositories since before rustic was a thing.

Only for one backend (restic server). It does NOT support append only for S3 or other backends which is the most frequently used scenario.

And some setups with rclone. How is that different from restic? The documented rclone setup doesn't support append-only in any meaningful way.

Sorry, I do not understand your comment. I will reiterate my statement. Use Rustic instead of restic.