You don't see a lot of grass bunkers on older courses because on most sites they did not expect to install a lot of drainage, so they didn't build any features which would have required it, other than sand bunkers.
I've never been a fan of grass bunkers because I wanted my courses to look old, and grass bunkers don't look old to me. We did make an exception on the new course in Scotland, where finding lots of little grass pockets in the ground is pretty standard. But, in general, where someone else might use a grass bunker, I'd just use a bit of contour (a small mound or a sharp dip, but not a real pocket) mowed at rough height.