target creature gets -x/-x until end of turn, where x is the number of times a capital letter appears in its rules text. (ignore reminder text and flavor text.)
capital offense has 1 printing, first in Unstable (2017). It has 2 official rulings.
Legality
commander: not legal
legacy: not legal
vintage: not legal
pauper: not legal
modern: not legal
pioneer: not legal
standard: not legal
brawl: not legal
Rulings
: all capital letters in the target creature’s rules text count, not just the ones in the first word of each sentence.
: ability words are part of the creature’s rules text. any capital letters an ability word has will count.