To answer the specific question, at 10.5 speed, the chart says you can cup up to 2.4 degrees reasonably, and up to 3.2 degrees, if you have that slope well around the cup. Past 3.4 degrees, it gets too steep. Multiply degrees by 1.75 to get % slope. So the max (according to this chart) is 4.2%, with up to 5.6% possible, but dangerous depending on all the other factors.
That chart was developed by ASGCA member Jerry Lemons (who contributes here) and was in the June 2008 USGA Green Section record. He did a fantastic chart showing max slope for different green speeds, and generally, they came in about the Masters/US Open range I describe below and more than most would call for in a "typical" recommendation. This reflects my personal experience that many are going too far in flattening greens.
At their tournaments the USGA has a system of measuring with a digital level over the cup in two directions. If the combined percent of those two was over 5, it was too steep if in the front of the green, and if in the back of the green, the combo measurements are 5.5 max, figuring not many players will be behind the pin and have a tougher downhill putt. BTW, the Pinehurst super told me that for the last Open there, with small greens they did not cup when those conditions were over 4.5 combined.
BTW, while you would think that these would come out at 2.75% and 2.25% slopes, they actually come out closer to 3.75% max and 3.2%. In fact, the USGA allows cups on slopes up to 4%, if there is 0% slope the other way.
Of course, this is for the national championship and for the best putters in the world. It makes sense that something less would be the max.
For "regular" courses, the USGA has never changed their recommendations of 3% max in cupping areas, despite increases in green speed since that recommendation was written. Ask a few green section reps privately and they nod in agreement that it could perhaps be revised downward. This suggests that drainage swales of 2-2.1% (a 2% swale with an additonal 2% side slopecomes out to about 2.8%,although I think its rare to have both slope components that steep) giving a max close to 3%) could still be acceptable.
I asked many gca's and most still seem to grade swales around 2% swales , with a steep green having swale slopes of perhaps 2.25%.
I know Hurdzan wrote of a range of 1.5-1.5%! Pete Dye once opined on this, and after converting from Dye to English, I concluded that his cup spaces areas max out at 2.25%. JN seems to be pretty flat, although I think TD has influenced him back up after Sebonic. Fazio seems to put real care in the slopes. I find that if he has some big rolls you need to putt over, the flat areas are really flat, even close to 0%. If a green doesn't have pronounced splines that you have to putt over, then the slopes can get over 4%. Really, his greens typically rock and roll!
I have recently reduced my base swales to under 2% based on complaints from golfers that my greens are just too d#*&*^ steep. When I play my courses at green speeds under 10, I think they play fine with swales at 2.25% and total slopes at under 3%, but it seems some golfers now dislike even having to aim way outside the hole on a long putt. I disagree, but I ain't the King!