What is the default Iranian Intermezzo game rule in CK3?

Enabled: In 867, the rulers of Persia, Afghanistan, and Iraq will be embroiled in a climactic Struggle set to decide the fate of the Abassid Caliphate and what remains of the Arabian Empire. (patch 1.20.1.2)

Does changing the Iranian Intermezzo rule disable achievements?

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

Game rule · Historicity · Game Modes

Iranian Intermezzo

Enabled · default
Disabled

Iranian Intermezzo is a game rule in Crusader Kings III (Historicity and Game Modes) with 2 settings: Enabled (the default) and Disabled. 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 Iranian Intermezzo game rule in CK3?: Enabled: In 867, the rulers of Persia, Afghanistan, and Iraq will be embroiled in a climactic Struggle set to decide the fate of the Abassid Caliphate and what remains of the Arabian Empire. (patch 1.20.1.2)
Does changing the Iranian Intermezzo rule disable achievements?: No: none of its 2 settings carries the flag blocks_achievements in patch 1.20.1.2.

2 settings

Enabled
Default
In 867, the rulers of Persia, Afghanistan, and Iraq will be embroiled in a climactic Struggle set to decide the fate of the Abassid Caliphate and what remains of the Arabian Empire.
struggle_persia_toggle_on
Disabled
The Struggle is not started when the game begins, meaning Persia, Afghanistan, and Iraq play like any other normal part of the map.
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 = struggle_persia_toggle_off in game/common and game/events, by the top-level object it sits in.
struggle_persia_toggle_off

Source: game/common/game_rules/00_game_rules.txt line 1248 (key struggle_persia), modifiers from game/common/modifiers, names from the English localisation; Crusader Kings III 1.20.1.2.