Karen,
Near as I can tell,
1950's - 6% typical
1960's - 4% typical
1970's - 3% common
1980's 2.5% common
1990's - 2% common
2000 - Sub 2% getting common.
All on typical courses, and not sure of typical green speeds much before the 1990's, where 10 was a lot.
I have measured tournament courses, and know the Masters and others use the old "digital scale in two directions" to determine acceptable slope at a cup location. Max is average of combined reading of 5 to 5.5 (higher at back of greens where fewer will be putting downhill) If you get a 3 and a 2 reading, or 5, and use the old Pythagoras theorem, that comes out to just less than 4% - Square root of 13 is 3.6%. Square root of 3 + 2.5 is 3.9%. Max is 4% in one direction and zero in the other. That is for 12-13 green speeds.
Of course, assuming that front part of ANGC was close to double digits, cross slopes and the length of run without much counter slope to stop the ball might still entice them to roll of the front of the green. Even at slower green speeds, it looks iffy, and I have to wonder if Mac knew exactly what slope he could get away with.