Distributions don't apply patches just for the sake of it or because we enjoy it -- it makes packaging more annoying for us when we have to rebase patches each version bump. But sometimes it's necessary and it would be a silly limitation to not allow yourself to patch software which is under a license that explicitly gives you permission to modify it.

I would argue any rolling-release or bleeding-edge distribution is pretty much the closest you'll get to "pure upstream". Stable distributions have more patches by necessity, and enterprise distributions even more so.