Do manufacturers specify any?

Depends on the bike, some do, most don’t.

Sometimes, depends on the frame/wheels and some don’t specifiy any. There is a rider in WSW& tri club on some Dura ace wheels and he weighs ~120kg.

Fulcrum say check your wheels reguarly if yo are over 85kg and as an aside a Whyte 901 MTB has a rider weight limit of 105 kg and that a Alu alloy frame.