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

Not Administrative: The Ghaznivids 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 — Kabulistan rule disable achievements?

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

Game rule · Historicity

Extra Administrative Realms — Kabulistan

Not Administrative · default
Always Administrative

Extra Administrative Realms — Kabulistan is a game rule in Crusader Kings III (Historicity) with 2 settings: Not Administrative (the default) 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 — Kabulistan game rule in CK3?: Not Administrative: The Ghaznivids 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 — Kabulistan rule disable achievements?: No: none of its 2 settings carries the flag blocks_achievements in patch 1.20.1.2.

2 settings

Not Administrative
Default
The Ghaznivids 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_kabulistan_gameplay_not_administrative in game/common and game/events, by the top-level object it sits in.
historicity_extra_admin_kabulistan_gameplay_not_administrative
Always Administrative
The Ghaznivids will always start with an Administrative Government 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_kabulistan_historic_always in game/common and game/events, by the top-level object it sits in.
historicity_extra_admin_kabulistan_historic_always

Source: game/common/game_rules/00_game_rules.txt line 1430 (key historicity_extra_admin_kabulistan), modifiers from game/common/modifiers, names from the English localisation; Crusader Kings III 1.20.1.2.