mkinitcpio VS bootmgr

This file has been changed recently and the wiki still references the old line. You have to modify this line now:

mkinitcpio is an arch thing: https://github.com/archlinux/mkinitcpio

The Arch Linux devs don't have infinite time and energy, so yes less-important issues that affect less users get diprioritized. You're welcome to PR the fix yourself, here's the mkinitcpio repo: https://github.com/archlinux/mkinitcpio

The general logic was changed in the new mkinitcpio release. https://github.com/archlinux/mkinitcpio/pull/54/files

That's an odd one since reinstalling the package fixes the issue. I wouldn't be so sure it's grub because it just loads the vmlinuz and initramfs that are suffixed with the pkgbase. That will not change based on the kernel version and pkgrel version. Someone brought up needing a hook. the mkinitcpio package already installs hooks and scripts necessary for maintaining a mkinitcpio prefix for each kernel for the initramfs image generation and installing vmlinuz to /boot/vmlinuz-${pkgbase} on upgrades. Everyone with the official mkinitcpio package is going to have the same /usr/share/libalpm/scripts/mkinitcpio-install shell script. Running grub-mkconfig is pointless for a kernel upgrade for the reason stated. All it does is export variables from /etc/default/grub and concatenates /etc/grub.d/* into a single config file. If there was a grub config issue it's unlikely you would ever be able to boot.

mkinitcpio made zstd the default on 2021-02-17

The number of thread is, but the compression level isn't. Because -3 is a sane default for the compression level in time of compression time and output file size, but this can differ for your machine. I find that on my machine best compression level is -15 or -16 for size and compression time. Since it will take a 1 second more to compress but it will yield a file 80% more compressed than the default. Here is the pill request where the defaults where changed mkinitcpio#47