Wearing Kilts!

Just some German games make the men remove their underwear. I think it's on the decline too.
Yeah, I’ve not heard of Scottish games having a rule requiring no underwear.

Some activities have specifically required underwear – though there has always been tonnes of people going commando in more relaxed events like Tug of War and similar. And pipe bands attending are usually dominated by true Scotsmen among the males. Always been plenty of visual interest under the kilts at the games I’ve attended 😁

But yeah, the only games I’ve heard absolutely mandating men go without underwear have been in Germany. Honestly I think it’s a great rule – nothing wrong with enforcing traditional attire!