And it's usually considered bad manners for men to be shirtless on the street, but it's not obscene.

It is absolutely haram in Islam, unless it is part of a religious function. Like when men (of one sect, I forget which) stand in the street in a large group and flog themselves, usually drawing blood & spraying it everywhere. They were allowed to do this in Toronto during the Covid-19.84 lockdowns, but Christians couldn't even meet in a parking lot, in their cars, 8 feet apart. That was banned, specifically. Muslims could! And continued holding prayers in mosques, iirc.