What is the default Legend Spread Chance game rule in CK3?

Normal: Legends will have a normal chance of spreading. (patch 1.20.1.2)

Does changing the Legend Spread Chance rule disable achievements?

No: none of its 5 settings carries the flag blocks_achievements in patch 1.20.1.2.

Game rule · Tweaks

Legend Spread Chance

Normal · default
High
Very High
Low
Very Low

Legend Spread Chance is a game rule in Crusader Kings III (Tweaks) with 5 settings: Normal (the default), High, Very High, Low and Very Low. 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 Legend Spread Chance game rule in CK3?: Normal: Legends will have a normal chance of spreading. (patch 1.20.1.2)
Does changing the Legend Spread Chance rule disable achievements?: No: none of its 5 settings carries the flag blocks_achievements in patch 1.20.1.2.

5 settings

Normal
Default
Legends will have a normal chance of spreading.
legend_spread_chance_normal
High
Legends will have a higher chance of spreading to adjacent provinces.
The script checks it once, in 1 place
Script values 1
- spread_chance_legends_mult_value
Every has_game_rule = legend_spread_chance_high in game/common and game/events, by the top-level object it sits in.
legend_spread_chance_high
Very High
Legends will have a much higher chance of spreading to adjacent provinces.
The script checks it once, in 1 place
Script values 1
- spread_chance_legends_mult_value
Every has_game_rule = legend_spread_chance_very_high in game/common and game/events, by the top-level object it sits in.
legend_spread_chance_very_high
Low
Legends will have a lower chance of spreading to adjacent provinces.
The script checks it once, in 1 place
Script values 1
- spread_chance_legends_mult_value
Every has_game_rule = legend_spread_chance_low in game/common and game/events, by the top-level object it sits in.
legend_spread_chance_low
Very Low
Legends will have a much lower chance of spreading to adjacent provinces.
The script checks it once, in 1 place
Script values 1
- spread_chance_legends_mult_value
Every has_game_rule = legend_spread_chance_very_low in game/common and game/events, by the top-level object it sits in.
legend_spread_chance_very_low

Source: game/common/game_rules/00_game_rules.txt line 464 (key legend_spread_chance), modifiers from game/common/modifiers, names from the English localisation; Crusader Kings III 1.20.1.2.