To think this will be an issue?

[lula99]

I’ve started seeing someone new. Both really into each other. I realised through social media that we have a mutual friend. An old school friend knows him from uni. We are both early 30s now.
I’m not particularly close to this old friend now as they have moved area and we drifted. I’m not sure if the guy I’m seeing is that close to him anymore either.
The issue is, years ago I slept with this friend. AIBU for worrying this new guy might sack me off for it if he finds out? Or do we think he won’t care?

He may be bothered if he finds out months down the line and you didn’t tell him. I doubt he’d be bothered if you told him now.

Frankly, if he’s bothered by who you slept with before your relationship started, that’s a bit of a 🚩 as might indicate possessive behaviour or ridiculous masculine “ownership” vibes.
You’re a grown adult with autonomy to sleep with whoever you like! It would be indiscreet to go blabbing about the notches on your bedpost though!

I'd just ask him about the guy and say "I went out with him once or twice, years ago. I haven't seen him since then" and leave it at that.