It is not super simple but I lived with outdated kernel and Xorg for a couple of years (because of Intel Poulsbo [1]). Process described in wiki [2] and uses virtualbox-host-modules as an example.
$ pacman -Qi glibc
Depends On : linux-api-headers>=4.10 tzdata filesystem
1. This version is not present in Arch Linux Archive anymore [3], search for package page [4], View Changes, search for corresponding PKGBUILD revision [5], makepkg. It is much easier if package is not that old and can be found in /var/cache/pacman/pkg/ or archive.
2. install with `pacman -U linux-4.15.8-1-x86_64.pkg.tar.xz`
3. skip package from being upgraded with /etc/pacman.conf IgnorePkg [6]
I had problems both with Ubuntu and Arch Linux updates. At least I can fix Arch Linux issues, Ubuntu felt broken.
But then you can't run leading edge hardware... My motherboard's network interface, wireless interface and my gpu all require a kernel version that's newer than the 19.10 ubuntu (and even 20.04 support for stability), let alone the last LTS release.
Ukku worked well for me to get newer kernels without issue, but kvm/virtualbox were totally borked, just as my hardware support was stable.
2. install with `pacman -U linux-4.15.8-1-x86_64.pkg.tar.xz`
3. skip package from being upgraded with /etc/pacman.conf IgnorePkg [6]
I had problems both with Ubuntu and Arch Linux updates. At least I can fix Arch Linux issues, Ubuntu felt broken.
[1] http://sergeykish.com/linux-poulsbo-emgd
[2] https://wiki.archlinux.org/index.php/Downgrading_packages
[3] https://archive.archlinux.org/packages/l/linux/
[4] https://www.archlinux.org/packages/core/x86_64/linux/
[5] https://github.com/archlinux/svntogit-packages/commits/packa...
[6] https://wiki.archlinux.org/index.php/Pacman#Skip_package_fro...