It’s possible to restore the /boot partition but it’s not easy as you’ll need to format the /boot partition to ext4 again, mount all Qubes OS partitions for chroot, download the kernel package from Qubes OS repository, copy it to your Qubes OS root partition, chroot into it, change UUID of /boot partition in grub.conf in EFI partition, in fstab and maybe somewhere else and then reinstall the kernel package (it should install vmlinuz and regenerate initramfs with dracut and regenerate grub config automatically).