What is the default Extra Administrative Realms — Maghreb game rule in CK3?

Not Administrative: The Maghreb will not have an Administrative Government in any start date, in line with expected gameplay. (patch 1.20.1.2)

Does changing the Extra Administrative Realms — Maghreb rule disable achievements?

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

Game rule · Historicity

Extra Administrative Realms — Maghreb

Not Administrative · default
Administrative post-1066
Always Administrative

Extra Administrative Realms — Maghreb is a game rule in Crusader Kings III (Historicity) with 3 settings: Not Administrative (the default), Administrative post-1066 and Always Administrative. 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 Extra Administrative Realms — Maghreb game rule in CK3?: Not Administrative: The Maghreb will not have an Administrative Government in any start date, in line with expected gameplay. (patch 1.20.1.2)
Does changing the Extra Administrative Realms — Maghreb rule disable achievements?: No: none of its 3 settings carries the flag blocks_achievements in patch 1.20.1.2.

3 settings

Not Administrative
Default
The Maghreb will not have an Administrative Government in any start date, in line with expected gameplay.
The script checks it once, in 1 place
Scripted effects 1
- apply_historic_administrative_game_rule_effect
Every has_game_rule = historicity_extra_admin_maghreb_gameplay_not_administrative in game/common and game/events, by the top-level object it sits in.
historicity_extra_admin_maghreb_gameplay_not_administrative
Administrative post-1066
The Maghreb will start with an Administrative Government in 1066 & 1178 to better reflect its historical organisation, though it may not function as well in play.
The script checks it once, in 1 place
Scripted effects 1
- apply_historic_administrative_game_rule_effect
Every has_game_rule = historicity_extra_admin_maghreb_historic_post_1066 in game/common and game/events, by the top-level object it sits in.
historicity_extra_admin_maghreb_historic_post_1066
Always Administrative
The Maghreb will always start with an Administrative Government, which is not reflective of its historical organisation in 867.
The script checks it once, in 1 place
Scripted effects 1
- apply_historic_administrative_game_rule_effect
Every has_game_rule = historicity_extra_admin_maghreb_alt_historic_always in game/common and game/events, by the top-level object it sits in.
historicity_extra_admin_maghreb_alt_historic_always

Source: game/common/game_rules/00_game_rules.txt line 1454 (key historicity_extra_admin_maghreb), modifiers from game/common/modifiers, names from the English localisation; Crusader Kings III 1.20.1.2.