User keeps seeing their comments, but other users cannot see their posts or their user page. Only Admins can shadowban. Mods can use a tool called automoderator to remove posts by certain users but only in their subreddit.
Was in /r/TIFU recently, too lazy to get the link.
Interestingly, one of them submits awful content. It's not spam but he is absolutely terrible. I wonder if they shadowban because the algo's figured out you suck.