The best objective evidence probably comes from Graham Kalton’s book on “The Public Schools”, which suggests prefects gave cp in a large minority of the 100-plus schools he surveyed. That was in 1965.
In my direct grant grammar school days (60s) slippering by prefects had only been ended a few years before – living memory so to speak – but we did have the rather fine alternative that prefects could actually take a first or second former to the Headmaster, with about a 50-50 chance that he would cane him. Only one or possibly two strokes, but it made a smart point. Even if he didn’t actually whack the junior miscreant, he would almost always threaten it and make a show of physically fetching out the cane. I think I got seven pupils whacked that way, in a year-and-a-bit as a prefect.