sysupgrade was released with OpenBSD 6.6. I think my wildest upgrade was going from 6.8 to current (7.3), so 5 releases without a hitch. Granted the box was mostly just base anyways. Always remember to "pkg_add -u" after every release.

The biggest change for me was when they added security to GPIO. I forget the details, but fabBSD(openBSD variant for CAMs) was a casualty last I checked.

It's a documented system what more could one ask for.