What is the default Extra Nomadic Region: Arabia game rule in CK3?

Default: The Arabian region is unchanged. (patch 1.20.1.2)

Does changing the Extra Nomadic Region: Arabia rule disable achievements?

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

Game rule · Historicity

Extra Nomadic Region: Arabia

Default
Nomadic

Extra Nomadic Region: Arabia is a game rule in Crusader Kings III (Historicity) with 2 settings: Default (the default) and Nomadic. 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 Nomadic Region: Arabia game rule in CK3?: Default: The Arabian region is unchanged. (patch 1.20.1.2)
Does changing the Extra Nomadic Region: Arabia rule disable achievements?: No: none of its 2 settings carries the flag blocks_achievements in patch 1.20.1.2.

2 settings

Default
Default
The Arabian region is unchanged.
arabia_nomad_region_off
Nomadic
The Arabian region uses Nomadic mechanics, designed for Steppe Nomads. This rule will not have any effect unless you own the Khans of the Steppe Expansion.
The script checks it once, in 1 place
On actions (the game's own hooks) 1
- on_game_start_after_lobby
Every has_game_rule = arabia_nomad_region_on in game/common and game/events, by the top-level object it sits in.
arabia_nomad_region_on

Source: game/common/game_rules/00_game_rules.txt line 1346 (key arabia_nomad_region), modifiers from game/common/modifiers, names from the English localisation; Crusader Kings III 1.20.1.2.