Many turntables just have a simple neon lamp for setting the speed of the platter. This blinks with the mains frequency.
Well, see this is where I'm going. If the neon strobe blinks with the mains frequency, and the turntable's speed itself is proportional to the mains frequency, then if the mains frequency changes down say 10%, then the strobe will strobe 10% slower and then maybe the turnable itself will spin 10% slower so I will not actually be able to tell except the music might sound funny. I doubt I would actually be able to tell because I used to have a much cheaper Marantz turntable with a synchronous motor, and it was 4% slow, and you really couldn't tell.