It was possible of course, but there are good reasons they were intrinsically linked:

The AC power line frequency was used for the vertical refresh rate for two reasons. The first reason was that the television's vacuum tube was susceptible to interference from the unit's power supply, including residual ripple. This could cause drifting horizontal bars (hum bars). Using the same frequency reduced this, and made interference static on the screen and therefore less obtrusive. The second reason was that television studios would use AC lamps, filming at a different frequency would cause strobing.

https://en.wikipedia.org/wiki/Refresh_rate#Televisions

You could also play PAL on 60Hz in the US with the right equipment. However, playing the video system on the opposite mains always had a weird flicker in the playback as the native power and playback just didn't line up.

It is similar to the effect that could be seen when using native lighting in a 50Hz mains country while recording video in NTSC. Not everyone would notice, but when you sit in an edit bay working with that footage all day, you notice everything.

Then again, some people don't notice the stuttering that is the result from 24->29.97 either. The modern 4th frame repeat drives me up the wall. </rant>

This explains why western Japan is so much further ahead than eastern Japan. Everything is just faster there.

There was a computer you could switch between 50 and 60 Hz with a button press. This controlled the CPU clock too so you got a faster computer with 60 Hz. But I can't remember what computer it was? Can't have been the Amiga or Atari even though they also could do it, has to have been an older one?

Are you thinking of the Turbo button on PCs?

I've personally never seen the ability to switch from mains frequency. That's a switch on the back of the PSU that typical goes from 240v or 120v.

The turbo button and mains frequency are 2 totally different things.

No, not the turbo button, it was the 50/60 Hz software switch that made the computer faster. Hm, might just have been that the gfx chip was working less in 50 Hz come to think of it...

I do remember seeing several computers in the 80s having this switch, but they were all modded. They also required you to have a more expensive monitor that could handle the format change (PAL/NTSC and 50/60hz)