I had a very similar situation with my ex right at the start of our relationship where he found old Facebook messages and posts from about 10 years before we met between me, one of my longtime male friends and a mutual female friend - there was never anything romantic between us, we were all part of a friend group from when we lived in the same city abroad.
Apparently he had been browsing my Facebook and I don’t know what he thought he saw or what they meant, but I woke up the next morning to about 10 missed calls and loads of massive long messages telling me how I was a liar, a cheat, and could never be trusted. I still to do this day don’t know what I’m supposed to have done.
He was angry that I hadn’t “disclosed” this past ex. He was never an ex, so there was nothing to “disclose.”
Honestly, when I think back now, it’s genuinely laughable and that’s exactly what I should have done and then blocked him forever, but I was an absolute wreck and was apologising for something that I still don’t understand.
These men are not right in the head - you have done nothing wrong, so please believe me when I tell you that.
This mental behaviour will only get worse. Please walk away now.
Edited to add: over the years, he also “stumbled across” other things on my various devices that made him react like that and accuse me of cheating. I had nothing to hide, so my devices where always lying around and he knew my passcode (I never knew his, but never wanted it anyway).
I didn’t actually give him my passcode - he heard me giving it to my mum to unlock and check something for me, so he memorised it and would check my phone, which really pissed me off, but I never changed the code because I was worried it would look suspicious to he next time he checked it.
I would also let him use my laptop for stuff he needed to do, and then of course, he would “stumble” across something.
To listen to him you’d think I led an extremely exciting life full of secret rendezvous and thrilling encounters - I’m actually boring as fuck and prefer being at home with my dog to anything else 😂.