Game concept
Natural Causes
The game also links it as Natural Cause.
From the game's encyclopedia
Passing on peacefully is the key to dying of Natural Causes. For this purpose, old age and chronic diseases are the main ways to leave this world.
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.
Natural Causes is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Passing on peacefully is the key to dying of Natural Causes." It is part of Character Death. The game also links it as Natural Cause. 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.