What is the default Black Death Frequency game rule in CK3?

Once: The Black Death can only occur once during your playthrough. (patch 1.20.1.2)

Does changing the Black Death Frequency rule disable achievements?

Yes for “Never”: the file gives it the flag blocks_achievements. The other settings keep achievements on.

Game rule · Difficulty · Health · Historicity

Black Death Frequency

Once · default
Unlimited
Never ✕

Black Death Frequency is a game rule in Crusader Kings III (Difficulty, Health and Historicity) with 3 settings: Once (the default), Unlimited and Never. “Never” 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 Black Death Frequency game rule in CK3?: Once: The Black Death can only occur once during your playthrough. (patch 1.20.1.2)
Does changing the Black Death Frequency rule disable achievements?: Yes for “Never”: the file gives it the flag blocks_achievements. The other settings keep achievements on.

3 settings

Once
Default
The Black Death can only occur once during your playthrough.
The script checks it once, in 1 place
Epidemics 1
- bubonic_plague
Every has_game_rule = bd_frequency_once in game/common and game/events, by the top-level object it sits in.
bd_frequency_once
Unlimited
The Black Death can appear an unlimited number of times in a campaign.
The script checks it once, in 1 place
Script values 1
- outbreak_chance_black_plague_mult_value
Every has_game_rule = bd_frequency_unlimited in game/common and game/events, by the top-level object it sits in.
bd_frequency_unlimited
Never
Disables achievements
The Black Death will never occur.
The script checks it once, in 1 place
Epidemics 1
- bubonic_plague
Every has_game_rule = bd_frequency_never in game/common and game/events, by the top-level object it sits in.
bd_frequency_never

Source: game/common/game_rules/00_game_rules.txt line 237 (key black_death_frequency), modifiers from game/common/modifiers, names from the English localisation; Crusader Kings III 1.20.1.2.