Agree Wilson did a great job with the back bunkers, with the floor at least a foot below the green, but they are also visible. He didn't worry too much about sand washing as we might today, and they have quite steep slopes above the flattish bottom.
My tendency is to make them visible and thus, level or above the green, with gentler slopes. Even then, they almost always require steeper slopes than frontal bunkers, because the top of a backing bunker that gets too far from the green usually looks awkward and disconnected. As Archie mentions, sometimes it does work, but it requires careful craftsmanship.
In general, I like the back bunkers smaller than the front ones, although I reverse that once in a while (Think Muirfield Village 13 (at least originally, may have changed) that had a small bunker front left and a long strip bunker along the back right. Larry Packard had a version of that long backing strip bunker in his repertoire too.
The tour pros I have worked with have often commented on the resulting downhill slope to a green generally falling away, and they don't like them. It is a hard shot, and maybe 10x for the average player, even if they don't know it, since their bar isn't to get the bunker shot a few feet from the hole as a pro might be.
It does seem like most gca's wrote about limiting them. Mac seemed to like them, but I feel it was because he was creating artful combos for artistic purposes more than anything.
In modern times, for public play many architects have espoused back left bunkers as a perfect way to make a course look hard, but play easy, on the theory that better players are the only ones who would miss long left, so they make a comeback for different reasons altogether.
Some believe in the idea of the "master hazard" and a backing hazard that is half as tough, a la the Eden Hole (although that is a front-side combo, not front right back left)
If cost conscious, building bunkers that rarely see play and only look good has probably diminished. They did seem more prevalent in the golden age and whatever you call the 1990-2005 era, but I haven't scientifically studied that.
Lastly, if you had your way, what would you think would be some "ideal" number of greens with back bunkers? I guess some sites, and some architects, who tend to build greens into up slopes naturally end of with more, but in my mind, 4-6 is more than enough.