It blocks the notifications, and it blocks the post from showing up in topic view. And it shouldn’t be possible for the user to tell they’re shadowbanned unless they try.
The list of shadowbanned users is stored in a setting with the usernames separated by |s. There’s probably a better way to do it, but I’m not sure how.
So, basically, a shadowbanned user’s post is invisible to all other users, but not to themselves. From their perspective, they are participating normally in the forum, but nobody ever responds to them. Is that correct? Can admins/moderators see their posts and reply to them privately within the forum, without needing to send a private message?