For experienced users who deal with card-mod for some time, a necessity to add the “card_mod” keyword is obvious.
Earlier a card-mod code usually worked w/o that “card_mod” keyword (it was only crucial for styling elements inside picture-elements card). But adding the “card_mod” keyword was recommended as a “good practice” for future compatibility.
That is why in Community (like this dedicated card-mod thread) people may find plenty of card-mod examples w/o this “card_mod” keyword; many of these examples may still work with added “card_mod” keyword (with a proper indentation).