He's blogged about it several times. According to his blog posts, he can track which posts get "shadowbanned" by blog analytics. Apparently, his posts regarding Trump or climate science aren't showing up in a lot of his followers' feeds. Even more remarkably, he seems to have gotten around it for now with code words - tweeting that the post is about kittens when it's really about a supposedly shadowbanned subject, which has supposedly restored his normal blog traffic rates from Twitter.

See http://blog.dilbert.com/post/156806516721/the-social-media-h... and http://blog.dilbert.com/post/157904840851/dopamine-puppets

Sure looks like Twitter is in the business of censoring content that doesn't fit their political views. If they'll do that, there's no telling what else they'll censor.