I have carried 20-30kg bags on board CX flights in all classes without problems. Obviously I do not put these in the overhead lockers.
Last year on one of my flights out of HKG I did see a CX agent try to gate check bags of economy passengers but this seemed to be a random process not based on weight or size.
Just out of curiosity, where do you stash away your 20-30kg luggage onboard ? Or is it just full of gold bars 😉🤭 n the size isn't like those on the Tokyo flights