Some of the modern-day countries retained their five-digit postcodes from Yugoslav times (Serbia and Bosnia for sure, maybe a few more, I'm too lazy to check), some only got rid of the first digit which used to identify individual Yugoslav republics (AKA modern-day countries).

So I'd say it's highly likely they'd be delivered, as it's still mostly the same, though I should point out many cities changed names since. For like the most basic example, Montenegro's capital was called Titograd between WW2 and 1992, before it swapped back to being called Podgorica.

I've encountered a surprising number of forms where "Serbia" isn't an option, but Yugoslavia is, even in 2026. There's been a number of times here in the Netherlands where I had to pick Yugoslavia as my place of birth on official government forms because we were technically still Yugoslavia in '98 and not Serbia and Montenegro.

I have no doubts that snail mail addressed to Yugoslavia still exists and probably gets routed just fine

Considering how my parents still refer to that area of the world as Yugoslavia, I'm pretty sure the postal system will know how to route it. Will probably be escalated to a human for labeling though.

There's hundreds of thousands of websites with the .su domain.

(The USSR dissolved before the world-wide-web was even a thing.)

If Barclays can get their own vanity TLD then Yugoslavia should be able too.