Possibly there are two separate issues here. The first issue is moderators doing their best to deal with problematic behaviour. Shadow banning obnoxious users was suggested as an approach for that here: Discourse Shadowban. I can understand the motivation, but I don’t think it will gain much traction as a moderation technique - if a user is that persistent, it’s just delaying an inevitable conflict.
The second issue is what’s being happening on other platforms where the reach of some user’s posts is limited, without giving the users any indication about what’s going on. What’s notable is that these users have not been in violation of the service’s TOS. It would be technically possible to create a Discourse plugin that would implement something like this, but unless the site was extremely active, I think it would be quickly discovered. I actually think that Discourse and other decentralized community platforms are the antidote to this issue. As an extreme example, if someone wants to create a community where saying “the earth revolves around the sun” is considered to be dangerous misinformation, they are free to do that. People with a heliocentric world view can just find a different community to join.
A further issue would be if it was impossible to find hosting, or a payment processor for a community that was going against the dominant narrative on a particular issue. Other than trying to promote the values of intellectual curiosity and diversity, I don’t have any great suggestions for how to deal with this.
Maybe, but also, coming up with solutions for real problems is a good business strategy. How to help people with their troll-type behaviour, as opposed to just banning them, feels like an unsolved problem. I can imagine Discourse providing some tools to help communities with this. It could also be outsourced to an external service that was staffed by trained therapists.