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
It would be logical if your date of birth changed the available options for country of birth to the set of countries that were contemporaneously recognized.
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.
Granted, ccTLDs has been already going on for years before USSR change their pronoun to were. Mostly for email, no idea if ccTLDs found their use on BBS.
I can understand .su continuing because Russia pretty much took over everything that represent Soviet Union elsewhere (embassies, Security Council seat, etc) and other former Soviet states either support the continuation or indifferent. Yugoslavia continuation is more contentious topic.
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.