Twitter itself started shadow banning me in the fall of 2017. My account had been vibrant up until then and suddenly no one saw anything I did or knew that I was there. You could start a new account, but they would catch you within three days.
Later they watched everything, any favorite or bookmark, any follow, and if they were in doubt, you got shadowbanned even if just lurking, based on what you read. It’s way more insidious than mentioned here. Everyone should avoid that thing.
I know that Disqus also shadow bans. They have admitted it.