Yes, I know it is in the CAA approved syllabus, but that doesn't mean it is smart for use in the context of giving the pilot more time to think.
Take a plane with Vs=50kt, Vy=70kt (a C152 perhaps?; years since I flew one).
Case 1. Flying at 55kt. Damn hard work, poor pitch control, poor forward vis, poor engine cooling. Not the sort of thing you would ever want to do in your own plane. 5kt above stall, 5kt above death if flying low.
Case 2. Flying at 70kt. Much easier and safer. Better control response etc etc.
The difference between the two, in terms of thinking time, is negligible, and I would suggest that Case 2 represents lower pilot workload and therefore more thinking time.
Slow flight is essential to teach as a part of the envelope which must be known, but to do it for giving the pilot more time to think... I don't think that's smart.