Nowadays, some climbing-fans see it as a challenge, and like to climb it. There have even been professional climbing classes held there in the past. Unless you're experienced, your author feels duty-bound to suggest you don't climb it, and just have a nice look at it. You climb very much at your own risk.
For more, see http://leabanksquare.blogspot.com/2008/06/mabley-green-boulder-by-john-frankland.html