A shadowban is IMO worse than an instaban, you still aren't able to post anything, but it will look like you can. It also feels morally wrong to shadowban people and there's no process to appeal, it's not even straightforward to discover you've been shadowbanned, you need a bot or searching for your posts in a private window[0].
Agree on the "simple way" of warning the user and putting the post on a queue though.