Automatic degraded mount at boot time isn't implemented by md either, it's implemented by a dracut module (or whatever creates the initramfs). The gist is that when mdadm assembly fails, the dracut module's script performs a 5 minute wait to see if all devices will appear, and then if not, tries to assemble degraded.

These days, with UEFI prevalent, a system won't boot without EFI system partitions replicated (and possibly sync'd) on every drive.