It is detrimental though. It is socially impolite to yawn in public.

Edit: why am I being downvoted for this?

Even if yawning in public affected sexual fitness: how long has it been socially impolite to yawn in public? Evolution takes a rather long time in species with long reproductive cycles. Almost all mammals yawn, it would take significant genetic changes to breed that out of us. That doesn't happen overnight.

> It is socially impolite to yawn in public.

No, it isn't. It can be socially impolite to yawn unexcused, when someone is talking to you, as it has come to be interpreted as boredom rather than tiredness or similar. But it isn't inherently impolite to, for instance, yawn when walking down the street, or in a setting where someone isn't talking to you.

I wonder if that has always been the case or if it is a modern thing (modern in the sense of our evolutionary history).

> why am I being downvoted for this?

Because you don’t know what detrimental means in this context and clearly don’t understand evolutionary timescales?

is it so detrimental that it leads to a person never finding a mate and reproducing? Maybe for a totally extreme outlier, but probably not