does the flatten brush in the recent version of blender replace the polish brush?
Do they do the same thing?
That's a really old brush!
In Blender 2.66 there was both a Flatten and a Polish Brush, but I can't find any difference between the two; they seem to do exactly the same.
I looked at the manual for that version, but in there the Polish Brush isn't even mentioned...
So I think you can safely use the Flatten Brush.
Flatten works but it's more of a "mushy" flattening behavior imo. I use the Scrape brush far more often now for hardsurface sculpting. See this lesson: https://cgcookie.com/lesson/sculpt-brushes-i-2-8?wtime=516