From the game's encyclopedia
Many Rites have Doctrines which designate certain behaviors as Shunned. Depending on the specific Doctrine, these behaviors may also be considered Criminal.
Characters who are caught practicing behaviors that are Shunned by their Rite lose a Level of Devotion and gain a corresponding Fame Trait; this Trait will be disliked by everyone whose Rite considers that behavior to be either Shunned or Criminal.
Examples of Shunned behaviors include:
• Fornicator
• Adulterer
• Deviant
• Sodomite
• Kinslayer
• Witch
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.
Shunned is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Many Rites have Doctrines which designate certain behaviors as Shunned." Patch 1.20.1.2, from game/common/game_concepts/00_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/00_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.