What is the default Historical Legends game rule in CK3?

Dynamic: Both historical Legend Seeds and Seeds generated by the in game events will appear. (patch 1.20.1.2)

Does changing the Historical Legends rule disable achievements?

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

Game rule · Historicity

Historical Legends

Dynamic · default
Only Historical Legends

Historical Legends is a game rule in Crusader Kings III (Historicity) with 2 settings: Dynamic (the default) and Only Historical Legends. 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 Historical Legends game rule in CK3?: Dynamic: Both historical Legend Seeds and Seeds generated by the in game events will appear. (patch 1.20.1.2)
Does changing the Historical Legends rule disable achievements?: No: none of its 2 settings carries the flag blocks_achievements in patch 1.20.1.2.

2 settings

Dynamic
Default
Both historical Legend Seeds and Seeds generated by the in game events will appear.
historical_legends_dynamic
Only Historical Legends
The only present Legend Seeds are those generated on game start, and no dynamic legends will appear.
The script checks it 38 times, in 23 places
Events 8
On actions (the game's own hooks) 8
- five_year_playable_pulse
- on_faith_created
- on_piety_level_gain
- on_raid_action_completion
- on_rite_created
- on_war_won_attacker
- on_war_won_defender
- random_yearly_playable_pulse
Scripted effects 3
Buildings 2
- Shrine
- Temple Keep
Activities 1
- Hunt
Scripted effects inside an event file 1
- finalise_combat_results_effect
Every has_game_rule = historical_legends_only in game/common and game/events, by the top-level object it sits in.
historical_legends_only

Source: game/common/game_rules/00_game_rules.txt line 492 (key historical_legends), modifiers from game/common/modifiers, names from the English localisation; Crusader Kings III 1.20.1.2.