Game concept
Legend Promoter
The game also links it as Legend Promoters.
From the game's encyclopedia
A Legend Promoter is a Ruler that supports another ruler's Legend.
A Legend Promoter gets extra bonuses that vary depending on the Legend Type and Legend Quality of the Legend in exchange for a monthly Gold contribution.
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.
Legend Promoter is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Legend Promoter is a Ruler that supports another ruler's Legend." It is part of Legend. The game also links it as Legend Promoters. Patch 1.20.1.2, from game/common/game_concepts/06_ce1_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/06_ce1_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.