Seems like even Arch maintains it's own kernel. Not uncommon for distros to do it at all.
What do you mean by 'maintain' here? Because Arch is trying to stay on the bleeding edge side of things in a reasonable manner and is rolling release, they almost stay 1:1 with upstream, "maintains its own kernel" makes it sound like they would do some heavy patching and do active maintenance with certain kernel versions, et cetera. You can have a look yourself if you want to https://git.archlinux.org/linux.git/commit/?h=v5.7.5-arch1&i...
https://i.imgur.com/jP6Pdvq.png
Also if you look at the diffstat you can see that there were a number of changes here:
https://i.imgur.com/J2em78v.png
It may be align with the majority of the kernel, but these patches exist in every version of the kernel that is released by Arch.
What else could that be called aside from maintaining your own kernel?
These changes are extremely small within scope which is the nature of Arch, but they still exist. A more hands-on distro will see many more changes than Arch, naturally. However, as evidenced from the above you can see that Arch does have it's own kernel and I cited Arch as an example because it strives so hard to maintain 1:1 with upstream.
Btw, I use Arch.
Archlinux doesn't do that, and straight merges Linux mainline.