It probably happens quite a few times? Mostly in lower-level matches imo. Afaik it's pretty damn rampant during a sort of qualifiers tournament in the China eSports LoL scene, which some current players were discovered to be a part of (I forgot their names, was just a part of some eSports news that I randomly watched). Ofc if caught, it's an instant fee or maybe even permanently banned from said tournaments and other future ones. So yes, it happens, but we can't exactly know if it does until revealed, if even revealed.

I don't expect the famous ones to be participating in it since they already earn much more so I don't think there's any reason or purpose to trying to risk their entire reputation for one match.