Hidden policy
From EverybodyWiki Bios & Wiki
A hidden policy, also known as undocumented or unwritten policy, is an unannounced, yet enforced policy against users of online community sites, video platforms and social such as Reddit.
Hidden policies prevent users from knowing in advance which banishable practices to avoid, and are commonly used to evade reasonable criticism from users without moderative or administrative powers, acting like a trap against them.