Android kernels 4.19 and higher should all have support included for WireGuard unless the OEM specifically disables it: [0]. The Pixel 8 ships with the android 14 6.1 kernel so it most definitely should have WireGuard kernel support. You can check this in the WireGuard app BTW, if you go to settings it will show the backend that's in use.
[0] https://android-review.googlesource.com/c/kernel/common/+/14...
Kernel support should have no bearing as the apps are purely userspace apps. You can use the kernel mode if you root the phone, but that's not a typical scenario.
Well, the issue isn't kernel vs user space, but you are correct that you still need a custom ROM and/or root unfortunately. I had assumed Android had also allowed netlink sockets for WireGuard but alas they did not. So the app can't communicate with the kernel module, bummer.