What is the default Random Nickname Frequency game rule in CK3?

Default: Random Nicknames appear on rulers at the default frequency. (patch 1.20.1.2)

Does changing the Random Nickname Frequency rule disable achievements?

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

Game rule · Tweaks · Flavor

Random Nickname Frequency

Default
Halved
Quartered
Disabled
Doubled

Random Nickname Frequency is a game rule in Crusader Kings III (Tweaks and Flavor) with 5 settings: Default (the default), Halved, Quartered, Disabled and Doubled. 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 Random Nickname Frequency game rule in CK3?: Default: Random Nicknames appear on rulers at the default frequency. (patch 1.20.1.2)
Does changing the Random Nickname Frequency rule disable achievements?: No: none of its 5 settings carries the flag blocks_achievements in patch 1.20.1.2.

5 settings

Default
Default
Random Nicknames appear on rulers at the default frequency.
random_nickname_frequency_default
Halved
Random Nicknames appear half as often.
The script checks it once, in 1 place
Script values 1
- random_nickname_chance
Every has_game_rule = random_nickname_frequency_halved in game/common and game/events, by the top-level object it sits in.
random_nickname_frequency_halved
Quartered
Random Nicknames appear one fourth as often.
The script checks it once, in 1 place
Script values 1
- random_nickname_chance
Every has_game_rule = random_nickname_frequency_quartered in game/common and game/events, by the top-level object it sits in.
random_nickname_frequency_quartered
Disabled
No random Nicknames will appear.
The script checks it once, in 1 place
Scripted effects 1
- assign_random_nickname_effect
Every has_game_rule = random_nickname_frequency_none in game/common and game/events, by the top-level object it sits in.
random_nickname_frequency_none
Doubled
Random Nicknames appear twice as often.
The script checks it once, in 1 place
Script values 1
- random_nickname_chance
Every has_game_rule = random_nickname_frequency_doubled in game/common and game/events, by the top-level object it sits in.
random_nickname_frequency_doubled

Source: game/common/game_rules/00_game_rules.txt line 826 (key random_nickname_frequency), modifiers from game/common/modifiers, names from the English localisation; Crusader Kings III 1.20.1.2.