What is the default The Great Steppe game rule in CK3?

Enabled: The Nomadic rulers of the Great Steppe will be involved in a Situation with its own Steppe Seasons and struggle to become the Gurkhan. (patch 1.20.1.2)

Does changing the The Great Steppe 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

The Great Steppe

Enabled · default
Disabled

The Great Steppe 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 The Great Steppe game rule in CK3?: Enabled: The Nomadic rulers of the Great Steppe will be involved in a Situation with its own Steppe Seasons and struggle to become the Gurkhan. (patch 1.20.1.2)
Does changing the The Great Steppe rule disable achievements?: No: none of its 2 settings carries the flag blocks_achievements in patch 1.20.1.2.

2 settings

Enabled
Default
The Nomadic rulers of the Great Steppe will be involved in a Situation with its own Steppe Seasons and struggle to become the Gurkhan.
situation_the_great_steppe_toggle_on
Disabled
The Great Steppe Situation is not enabled when the game begins, meaning that the Steppe plays 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 = situation_the_great_steppe_toggle_off in game/common and game/events, by the top-level object it sits in.
situation_the_great_steppe_toggle_off

Source: game/common/game_rules/00_game_rules.txt line 1226 (key situation_the_great_steppe), modifiers from game/common/modifiers, names from the English localisation; Crusader Kings III 1.20.1.2.