I agree that initrd integrity protection would be an important security improvement against evil maid attacks.
However /boot encryption does not actually prevent the equivalent of initrd tempering, it only prevents one variant.
You see with an encrypted /boot, as an attacker with disk access I can just as easily modify your ESP where your GRUB lives and modify GRUB’s initial bootstrap configuration (on the ESP) to still decrypt your /boot partition, but not actually load the initrd from there but load a malicious initrd from an unencrypted location (e.g. the ESP). You will be none the wiser in either case and the exact same attack was accomplished.
The only solution to this issue is the bootloader to actually enforce initrd signing, /boot encryption and even optional initrd signing is a completely inadequate defence.