You can work around this by sticking rclone in the middle which does support soft delete: https://pricey.uk/blog/restic-backups-without-delete/

In short, give rclone your keys, (small ~/.rclone config file) tell restic to use an rclone backend and add an extra argument when running restic. (See the blog link above, all explained)

There's an open issue for this which doesn't seem to have moved: https://github.com/restic/restic/issues/2134 - it does have a couple of PRs linked on there which implement proper soft-delete.