What is the default Historical Characters game rule in CK3?

Default: Various historical characters such as Thomas Aquinas, Ibn Battuta, or Hildegard von Bingen can spawn. You will only be notified if they are born in your Domain or if they are very well-known. (patch 1.20.1.2)

Does changing the Historical Characters rule disable achievements?

No: none of its 3 settings carries the flag blocks_achievements in patch 1.20.1.2.

Game rule · Historicity

Historical Characters

Default
Notify All
Disable Spawns

Historical Characters is a game rule in Crusader Kings III (Historicity) with 3 settings: Default (the default), Notify All and Disable Spawns. None of its settings disables achievements. Patch 1.20.1.2, from game/common/game_rules/00_game_rules.txt.

This section as JSON: /raw/game-rules.json

Quick answers (2)

What is the default Historical Characters game rule in CK3?: Default: Various historical characters such as Thomas Aquinas, Ibn Battuta, or Hildegard von Bingen can spawn. You will only be notified if they are born in your Domain or if they are very well-known. (patch 1.20.1.2)
Does changing the Historical Characters rule disable achievements?: No: none of its 3 settings carries the flag blocks_achievements in patch 1.20.1.2.

3 settings

Default
Default
Various historical characters such as Thomas Aquinas, Ibn Battuta, or Hildegard von Bingen can spawn. You will only be notified if they are born in your Domain or if they are very well-known.
default_historical_characters
Notify All
Various historical characters such as Thomas Aquinas, Ibn Battuta, or Hildegard von Bingen can spawn. You will be notified of everyone born within your entire sub-Realm.
The script checks it once, in 1 place
Scripted effects 1
- spawn_historical_characters_boilerplate_effect
Every has_game_rule = all_historical_characters in game/common and game/events, by the top-level object it sits in.
all_historical_characters
Disable Spawns
No historical characters will spawn.
The script checks it once, in 1 place
On actions (the game's own hooks) 1
- yearly_global_pulse
Every has_game_rule = no_historical_characters in game/common and game/events, by the top-level object it sits in.
no_historical_characters

Source: game/common/game_rules/00_game_rules.txt line 74 (key historical_characters), modifiers from game/common/modifiers, names from the English localisation; Crusader Kings III 1.20.1.2.