The one in 3.1 just flattened stuff, like it’s name… The new one builds geometry or punches a hole and at the same time flattens that new surface. 
Supposedly they added a Flatten brush that is more akin to the flatten in 3.1 . I’m not sure which one it is… “Flatten B” maybe?
In any event, you really can’t . The new brushes just behave differently, and I dont think you can get an exact match.
But I encourage you to look at the new Polish brushes instead. They are far more versatile and intuitive “flatten” brushes than the old ones ever were. Ill never touch the old Flatten brushes again.
And if you want something that flattens even more absolutely, check out the trim brushes, in particular the Trim Dynamic for its freehand behavior.
Ty. The new brushes are good, there are some very useful ones. I just kinda liked the simplicity and fluidity of the ones in Z 3.1, they are still there but 3.1 had smoother less dotted strokes. That’s the only real issue I have with 3.5, the dotting.
In that case, that is an issue common to all the brushes in 3.5, not just the Flatten… an unavoidable consequence of the new brush system (which offers in an overall sense, many more benefits). You can can improve it to a moderate degree by going to Preferences> Performance>Mouse and Tablet samples, and turning that all the way up. It will make a significant improvement, though there will still be an adjustment period while adapting to the optimal way to work with the new brushes.