Hi,
I just signed up so I could post this (as my humble contribution to this fantastic project):
I just bought a (relatively) new Pixel 8 Pro via ebay. It was factory reset and did not have a sim, net provider or any other lock but "OEM unlocking" was still greyed out (after activating the developer menu).
I tried every tip I found (reset, factory reset, update, ...). Nothing worked.
I contacted Google but they only told me to try these things (that I had already tried) and that they could not unlock OEM. Trying to figure why I couldn't unlock it myself didn't get me anywhere. They just kept confirming that they couldn't do it for me but never answered my question.
And this is how I solved it: My phone was, as I said, used. The previous owner had enrolled it in the Android Beta program. After removing it from the Beta program and reinstalling the stable Android release (and activating the developer menu again), OEM unlocking was no longer greyed out but became available.
So either you can't OEM unlock while in Beta or it needed a full reinstall and not just some minor update to activate OEM unlocking.
I hope this will help others.