Appearance Modifier and related tokens should permit negative numbers.

BP_APPEARANCE_MODIFIER, APP_MOD_DESC_RANGE, and possibly BODY_APPEARANCE_MODIFIER.

They all complain:

This parameter does not accept values smaller than 0. DF RAW Language Server(too_small_int)

While negative values aren't used in the vanilla raws the game does accept them and they work as expected for all except BODY_APPEARANCE_MODIFIER.

BODY_APPEARANCE_MODIFIER differs in that the number is a percent used to modify the creature's BODY_SIZE. When using numbers <= 0 the game uses some minimum value for the creature size instead.