What is the default Fall of Al-Andalus game rule in CK3?

No Limitation: The Dissolution Faction can hit the Kingdom of Al-Andalus from the start of the game. (patch 1.20.1.2)

Does changing the Fall of Al-Andalus rule disable achievements?

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

Game rule · Historicity · Flavor

Fall of Al-Andalus

No Limitation · default
Death of the Caliph
Historical Timing

Fall of Al-Andalus is a game rule in Crusader Kings III (Historicity and Flavor) with 3 settings: No Limitation (the default), Death of the Caliph and Historical Timing. 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 Fall of Al-Andalus game rule in CK3?: No Limitation: The Dissolution Faction can hit the Kingdom of Al-Andalus from the start of the game. (patch 1.20.1.2)
Does changing the Fall of Al-Andalus rule disable achievements?: No: none of its 3 settings carries the flag blocks_achievements in patch 1.20.1.2.

3 settings

No Limitation
Default
The Dissolution Faction can hit the Kingdom of Al-Andalus from the start of the game
dissolution_unconstrained
Death of the Caliph
The Dissolution Faction can only hit the Kingdom of Al-Andalus after the death of Sultan Muhammad $Abd-al-Rahman$
The script checks it once, in 1 place
Scripted modifiers (AI weights) 1
- nation_fracturing_faction_blockers
Every has_game_rule = dissolution_after_caliph_death in game/common and game/events, by the top-level object it sits in.
dissolution_after_caliph_death
Historical Timing
The Dissolution Faction is only activated after the year 1020 for the Kingdom of Al-Andalus
The script checks it once, in 1 place
Scripted modifiers (AI weights) 1
- nation_fracturing_faction_blockers
Every has_game_rule = dissolution_historical_date in game/common and game/events, by the top-level object it sits in.
dissolution_historical_date

Source: game/common/game_rules/00_game_rules.txt line 1197 (key dissolution_umayyad), modifiers from game/common/modifiers, names from the English localisation; Crusader Kings III 1.20.1.2.