Don't take my word for it, read the related documentation[1]:

"The Fedora Project focuses, as much as possible, on not deviating from upstream in the software it includes in the repository. The following guidelines are a general set of best practices, and provide reasons why this is a good idea, tips for sending your patches upstream, and potential exceptions Fedora might make. The primary goal is to share the benefits of a common codebase for end users and developers while simultaneously reducing unnecessary maintenance efforts."

The linked[1] documentation also answers, with specific examples, the question of: "What are deviations from upstream?"

[1] https://fedoraproject.org/wiki/Staying_close_to_upstream_pro...

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.

> Arch [Linux] strives to keep its packages as close to the original upstream software as possible. Patches are applied only when necessary to ensure an application compiles and runs correctly with the other packages installed on an up-to-date Arch system.