First of all, sleepers (generally vertical wooden supports for the faces of bunker and such) were definitely a physical feature and style of an era---eg generally from 19th century architecture and beginning their use and existence abroad.
Macdonald, with his creation of NGLA (1907-11) and with its list of approximately a half dozen 19th century template holes took the look and style and physical architectural features from those holes abroad and recreated them at NGLA, and that originally included wooden sleepers.
Macdonald also continued to constantly work on changing and improving NGLA for about the next 30 years and during that time Macdonald removed those wooden sleepers from the 6th green as well as redoing some greens or parts of them and even moving a few others.
Also, probably the continuous use of sand topdressing may tend to flatten out or soften some contours, particularly low or concave spots, over time. On the other hand, some green contours immediately surrounding sand bunkering may tend to rise up in height through constant sand kick (virtually a player form of constant sand topdressing).
The speeds on the old green in that photo were probably about half as fast as the greens of NGLA today so my sense is that the contours of the 6th green, even if somewhat softer, probably function in play as far more dramatic and more demanding than the contours on that old green did.
Back to wooden board sleepers.
Even the old redan at North Berwick once had board sleepers supporting the face of its big fronting bunker. It now has a grass face, as does NGLA's #6.
For a time in the early years some courses such as Pine Valley and even NGLA tried to support some of the more vertical faces of bunkering just using sand. Basically, that just didn't work---it never held for long and would just wash down often taking parts of a green with it.
At that point almost all of the courses who used wooden sleepers went to grass faces or even smaller more formalized bunkers in place of a big single sand flashed up one.
And abroad on most of the steep faced bunkers that once used wooden sleepers for structural support bunkers were refitted with "sod courses" that became known as revetting.
I can tell you that today if a ball hits the front of the 6th green near and certainly to the left of that golfer at the front of the green it will generally roll off the green into that left bunker. And I can tell you that if you are outside the horseshoe and the pin is in it it's very hard to get the ball near the pin and often even quite hard to keep the ball in the horseshoe itself.
I can also tell you that green with today's firmness and speed on NGLA's greens is basically three greens in one (the horseshoe, the right portion and the back left portion) and if you are in the wrong section to the pin the chances of two putting are very low.
In other words, for what NGLA is looking for today playability-wise, the 6th green and everything about it is just fine!