From the game's encyclopedia
Married Spouses can arrange a Divorce, although the rules, requirements and consequences depend on the Doctrines of their Faiths.
Concubines can simply be set aside at will.
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.
Divorce is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Married Spouses can arrange a Divorce, although the rules, requirements and consequences depend on the Doctrines of their Faiths." The game also links it as Divorced, Divorcing and Divorces. 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.