Animal Crossing: New Horizons' most recent update allows islanders to visit dream islands of other players, but Nintendo wants to make sure that those dream islands aren't up to any funny business.
Multiple players have reported that their dream islands have been deleted due to hacked content. Nintendo is sending emails to those players letting them know that their dream islands were removed due to violating the Nintendo Account User Agreement. While the hacks may technically not be hurting anyone, Nintendo doesn't want players visiting hacked dream islands in general.
If you're planning on letting people visit your dream island, you better make sure you're playing as intended!