I don't know when they made the change, but I see that older UPS use large heavy 50/60HZ transformers the newer ones seem primarily to use a switching mode type setup which is smaller and more efficient.
I saw something at one point that said the modified sine or switch mode style didn't work well with things that have active power factor correction. I don't even know for sure what that does, but the few things I think had it were all like from the late 90s- early 2000s. Usually I can tell looking through the vents or seeing screws in the PSU case that aren't holding the case together or some fan, etc in. with screws that must be holding some part. What I think are PFC units have a dual winding transformer like thing screwed to the case.
To me I can see almost no reason anymore to use a 50/60hz UPS. Unless you have ancient hardware or...