When the humanity learned to speak, it also learned there are people, who should just shut up. In our digital age, we deal with misbehaving people by banning their account. Bans are often temporary and tell the user what they did wrong, so that they can learn from their mistakes. Sometimes they do not learn and the ban becomes permanent. However, they can always just create a new account, using different IP address and temporary email, so they can bother everyone until they are banned again.
Introducing the shadowban. Do not say anything, just let the user log in and comment as usual, but hide the account’s activity from everybody else. The hope is that they will leave for good, because everybody is ignoring them. Well, that might have worked two decades ago, but people now know what is up. When you get reactions regularly, but your last few comments are ignored, you get suspicious. Then you just need to log out and check whether you can still see your comments. If not, you can be sure you have been silently banned. So, compared to a regular ban you might just get an extra day or two of silence, but the person will never correct their behavior, because you did not tell them what they did wrong.
Shadowbans can still work though, but it requires more effort. Instead of no engagement you would have to fake it, so that the banned person will take longer to notice a difference. After all, they are still getting likes, so everything is normal, right? You could even fake comments using a chatbot. However, is it really worth it to spend all this effort on a few individuals? You do not want their spam, yet you are still keeping track of it.
The reason ban exists is so that a community can get rid of someone, who does not fit. A community is able to police itself, we already have the tools. You want to ignore someone? Put them in your ignore list, hiding everything they do. Someone has to go? Enough users report them and they are gone. There is no need for shadowbans, possibly not even moderators. So what that the spammer keeps coming back? When every user has a fraction of the power needed to cause a ban, the spammer will soon be gone again. Worried about someone creating dozens of accounts to single-handedly ban people? Simply give the power to accounts only after they had enough activity. You did nothing wrong, yet other people voted to ban you? Then you are in the wrong community, go find or create a better one.