We don't have any policy. Sticking with upstream is a shared value between the packagers but it's important to note that we generally don't enforce any policy. Most packages has no patches. Usually it's regressions or security patches if there is anything.

Current linux release has one patch changing one default: https://github.com/archlinux/linux/commits/v5.9.8-arch1

The wording on this page suggests that it is a policy:

https://wiki.archlinux.org/index.php/DeveloperWiki:Patching

As you say, the page notes that "[the] policy is intended to suggest, not to enforce", but having a policy is orthogonal to enforcing it.

And as a packager for Arch the past 3 years: I had no clue this page existed. Evidently we are bad at these policy things. But I'd rather call them social norms then packaging policies.

Even unspoken or badly specified policies can be policies. Just unwritten ones in that case. I thoroughly enjoy this one.

Or why are other packages patched? Is it because it takes long to accept them at upstream?

But why is kernel is patched by distros at all? I run kernel from kernel.org always and don't see any issues.

On arch, patches are usually done to either customize the build version (the kernel is 5.9.xy-arch1 for example, that's the only patch), or to make them build with the newer compiler and libs present on the system. Additionally any patches necessary to make them work at all, though in my experience rare.