As linux-lts went to version 5.10, all official kernels of Arch Linux now accept initramfs zstd compressed images, so mkinitcpio defaults to zstd compressed images with version 30, which is currently being tested.
If you are using a kernel version before 5.9, be sure to change mkinitcpio.conf
COMPRESSION
to use one of the supported compressions, such as gzip, otherwise you will not be able to start images generated by mkinitcpio.