If a user blocked on a wiki try to edit, they get a notice. Notices are not helpful and can cause more problems with block evasion, sock puppetry, etc.
Shadow banning ensures that a blocked editor leaves the site by boredom instead of trying to evade blocks and becoming a long term abuser.
The idea is simple: don't communicate in any way that they are blocked. (Even hide the block log entry from the blocked user!) Edits that they make to pages should tell the user that their edit is successful, but the edits should be invisible to all users, including administrators. The edits can be stored as a browser cookie instead of on the server. Eventually, the editor will get so bored that the community is ignoring them that they will give up and leave.
PS I posted the idea on meta initially. Check the bottom of this page: https://meta.wikimedia.org/wiki/Talk:Community_health_initiative/Blocking_tools_and_improvements