What is the default Custom Kingdoms game rule in CK3?

On: The creation of custom non-historical Kingdoms and Empires is allowed. (patch 1.20.1.2)

Does changing the Custom Kingdoms rule disable achievements?

No: none of its 2 settings carries the flag blocks_achievements in patch 1.20.1.2.

Game rule · Historicity · Titles & Realm

Custom Kingdoms

On · default
Off

Custom Kingdoms is a game rule in Crusader Kings III (Historicity and Titles & Realm) with 2 settings: On (the default) and Off. 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 Custom Kingdoms game rule in CK3?: On: The creation of custom non-historical Kingdoms and Empires is allowed. (patch 1.20.1.2)
Does changing the Custom Kingdoms rule disable achievements?: No: none of its 2 settings carries the flag blocks_achievements in patch 1.20.1.2.

2 settings

On
Default
The creation of custom non-historical Kingdoms and Empires is allowed.
on_custom_kingdoms
Off
The creation of custom non-historical Kingdoms and Empires is disallowed.
The script checks it 4 times, in 4 places
Decisions 3
Character interactions 1
- Consecrate Kingdom
Every has_game_rule = off_custom_kingdoms in game/common and game/events, by the top-level object it sits in.
off_custom_kingdoms

Source: game/common/game_rules/00_game_rules.txt line 995 (key custom_kingdoms), modifiers from game/common/modifiers, names from the English localisation; Crusader Kings III 1.20.1.2.