Admittedly, it may be my disgust talking.

I can see where the same argument could be used to force everyone to wear a mask because breathing transfers germs, which I wouldn't want.

At the same time, loose fluid, even when it isn't disease-carrying, inspires significantly stronger reactions in many people, some even pass out at the sight of blood. Even the smell (or sound) of vomit can inspire others to vomit reflexively.

I've never heard of anyone passing out at the sight of someone breathing. (If you have superior knowledge here, I'm all ears.)

I also grant considerations for population density. It wouldn't matter on the Appalachian Trail, as an example. Have at it.

Yet, as animals, we do have psychological frailties that have to be accounted for as a condition for cities (large-scale cooperation) to function. And these lines may be drawn differently where cultural norms of disgust vary.

Imagine the Tokyo subway in a world with copious human juice publically flaunted.

Sure, some people would be into that, and maybe you could have a city or two where that would be the norm, but trading with said group might be a non-starter. Otherwise, juice on?

On a broader scale, I don't think flagrant human juice wearing would be functional for a large majority of people living and working in close quarters with strangers. Caveat, I haven't run the experiment, and I strongly don't want to be part of such an experiment.

Where to remove the restriction as populations dwindled? I don't know, but not too close to me personally would be great.