Game concept
Prohibited
The game also links it as Prohibit.
From the game's encyclopedia
A Prohibited Tenet Status means the Rite explicitly forbids this Tenet or Doctrine. A Character of the Rite adopting the Tenet as a Personal Tenet keeps it as a Secret, and a Rite adopting Prohibited Tenets as Core becomes much more Divergent from its Faith.
The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.
Prohibited is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Prohibited Tenet Status means the Rite explicitly forbids this Tenet or Doctrine." It is part of Tenet Status. The game also links it as Prohibit. Patch 1.20.1.2, from game/common/game_concepts/pam_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/pam_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.