A shadow ban occurs when an online publisher enables you to see your own content but hides it from everyone else. A consequence of a shadow ban is that the person who has been banned may be unaware that their content is no longer visible.
This is not new – indeed, Twitter has been caught shadow banning in the past and now admits they do it. Twitter, like most social media, usually says this was due to an erroneous flag or software error. My own posts – innocuous ones – have been shadow banned on Twitter, Facebook and by the Oregonian newspaper.