What is the default Diplomatic Range game rule in CK3?

Default: The diplomatic range is unchanged. (patch 1.20.1.2)

Does changing the Diplomatic Range rule disable achievements?

Yes for “Unrestricted”: the file gives it the flag blocks_achievements. The other settings keep achievements on.

Game rule · Tweaks · Historicity

Diplomatic Range

Default
Restricted
Unrestricted ✕

Diplomatic Range is a game rule in Crusader Kings III (Tweaks and Historicity) with 3 settings: Default (the default), Restricted and Unrestricted. “Unrestricted” 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 Diplomatic Range game rule in CK3?: Default: The diplomatic range is unchanged. (patch 1.20.1.2)
Does changing the Diplomatic Range rule disable achievements?: Yes for “Unrestricted”: the file gives it the flag blocks_achievements. The other settings keep achievements on.

3 settings

Default
Default
The diplomatic range is unchanged.
default_diplomatic_range
Restricted
Restricted diplomatic range
The diplomatic range is further restricted, making non-local interactions significantly more rare.
restricted_diplomatic_range
Unrestricted
Turns off diplomatic rangeDisables achievements
Diplomatic range is turned off, allowing anyone on the map to interact with anyone else.
unrestricted_diplomatic_range

Source: game/common/game_rules/00_game_rules.txt line 921 (key diplomatic_range), modifiers from game/common/modifiers, names from the English localisation; Crusader Kings III 1.20.1.2.