The idea is that the person being shadowbanned is either communicating in bad faith (trolling or trying to underhandedly manipulate others), or is a paranoid schizophrenic.
In practice, though, shadowbans are applied to people who are persistent annoyances, even if they're not intentionally so, and even if they're not delusional. The result is a shift of the forums' Overton Window. Gradually, other people become annoying outlyers on contentious topics, and they have to be shadowbanned as well to "keep the peace". On and on it goes.
People who are justifiably shadowbanned — trolls and paranoids — probably check periodically to see if their main account has been shadowbanned. Otherwise they're poor trolls or not very paranoid paranoids. If a troll discovers this, they will be enraged and redouble their efforts if it's technically possible to get around the ban. If a paranoid discovers a shadowban, it will feed their paranoia. Either case is very bad for the forum, when the toxic user inevitably re-registers (assuming they're able; if they're not why couldn't they just be banned outright?), causing chaos until they're identified and shadowbanned again. The only person this really works on is the unintentionally annoying poster who discovers the shadowban and becomes depressed over being gaslighted, and might leave the forum on account of that.
Shadowbans seems like they're part of a war of attrition against justifiable targets, with quite a lot of collateral damage.
> If a troll discovers this, they will be enraged and redouble their efforts if it's technically possible to get around the ban.
In my experience shadow-banning people on my forum, outright banning someone tends to enrage people much more since they basically realize they are banned in the heat of the moment that got them banned. One benefit of shadow-banning is that they may have cooled off by the time they realize, and frankly people tend to handle it better, even sometimes a "ah, touche" sort of mentality.
I've noticed my own posts on HN individually shadow-banned by looking at my comment history in incognito mode and I get it. "Yeah, I knew I shouldn't have worded that post so strongly."
Better mod processes help the issue. On my forum, people shadow-banned pop up in review queue at intervals. If they seemed to have cooled off, they can be unbanned. Mods can also vouch for certain posts like people can on HN with showdead on. There's a view in the mod panel that sorts people by vouch count and we can potentially unban people that way as well.
Nobody would need a shadow-banning mechanism if the moderators had unlimited resources. People forget that it's an effort to be on the right side of the trapdoor function.
Meanwhile I wager that people that don't think shadow-banning is ever just are coming from an idealism you tend to have before you actually try running a forum and realize the extent a single person can harm your community and waste everyone's finite time.
I think the next level of shadow banning (if not happening today) will be shadow supporting or shadow questions/responses. Where an entire thread keeps progressing to convince a banned user from investigating their true status by keeping the dopamine flowing.