Yep, blast from the past.
Yes, fighting with a handgun, which includes revolvers, has evolved a bit. However, the basics of the old FBI taught revolver course would still stand someone in good stead as a foundation.
Having been both a SA and DA revolver shooter, I still tend to use a curled master hand thumb. The job of the left thumb is to not interfere with anything (and to cock a SA revolver's hammer when shooting SA wheelies).
I've watched my fair share of shooters experience a gouged/bleeding thumb when they elevated their master hand thumb too high alongside the frame, and the rear of the cylinder came back and clipped it under recoil. Varies by the individual, of course, as well as the size of the wheelgun. One guy somehow also managed to have the cylinder release cut his thumb knuckle when he let his thumb ride too high.
Personally, I never cared whether someone wanted to use a neutral/high thumb hold on their revolvers, as long as it didn't interfere with their shooting ... and ... it didn't adversely affect their 1-handed shooting by weakening their control. It's hits going where intended that matter, along with sufficient controllability under recoil, after all. No prizes for appearance.