What is the default Unrestricted Dynasty Legacies game rule in CK3?

Default: Dynasty Legacy tracks use their normal cultural, governmental, and regional requirements. (patch 1.20.1.2)

Does changing the Unrestricted Dynasty Legacies rule disable achievements?

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

Game rule · Game Modes · Tweaks

Unrestricted Dynasty Legacies

Default
Player Only
All Rulers

Unrestricted Dynasty Legacies is a game rule in Crusader Kings III (Game Modes and Tweaks) with 3 settings: Default (the default), Player Only and All Rulers. 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 Unrestricted Dynasty Legacies game rule in CK3?: Default: Dynasty Legacy tracks use their normal cultural, governmental, and regional requirements. (patch 1.20.1.2)
Does changing the Unrestricted Dynasty Legacies rule disable achievements?: No: none of its 3 settings carries the flag blocks_achievements in patch 1.20.1.2.

3 settings

Default
Default
Dynasty Legacy tracks use their normal cultural, governmental, and regional requirements.
unrestricted_dynasty_legacies_default
Player Only
Player Rulers can view and unlock every Dynasty Legacy track, ignoring normal cultural, governmental, and regional requirements. DLC requirements still apply.
The script checks it 11 times, in 11 places
Dynasty legacies 11
- ep3_administrative_legacy_track
- fp1_adventure_legacy_track
- fp1_pillage_legacy_track
- fp2_coterie_legacy_track
- fp2_urbanism_legacy_track
- fp3_khvarenah_legacy_track
- mpo_nomad_legacy_track
- pam_legacy_track
- tgp_china_legacy_track
- tgp_japan_legacy_track
- tgp_southeast_asia_legacy_track
Every has_game_rule = unrestricted_dynasty_legacies_player_only in game/common and game/events, by the top-level object it sits in.
unrestricted_dynasty_legacies_player_only
All Rulers
All Rulers can view and unlock every Dynasty Legacy track, ignoring normal cultural, governmental, and regional requirements. DLC requirements still apply.
The script checks it 11 times, in 11 places
Dynasty legacies 11
- ep3_administrative_legacy_track
- fp1_adventure_legacy_track
- fp1_pillage_legacy_track
- fp2_coterie_legacy_track
- fp2_urbanism_legacy_track
- fp3_khvarenah_legacy_track
- mpo_nomad_legacy_track
- pam_legacy_track
- tgp_china_legacy_track
- tgp_japan_legacy_track
- tgp_southeast_asia_legacy_track
Every has_game_rule = unrestricted_dynasty_legacies_all in game/common and game/events, by the top-level object it sits in.
unrestricted_dynasty_legacies_all

Source: game/common/game_rules/00_game_rules.txt line 1648 (key unrestricted_dynasty_legacies), modifiers from game/common/modifiers, names from the English localisation; Crusader Kings III 1.20.1.2.