The ESP does not need to contain /boot, it only needs to contain the EFI. And the UEFI specifications are for an independent filesystem based on FAT, VFAT is still patented. In practice, any FAT filesystem usually works.
You actually don't need a separate /boot - recent grub versions know what to do. You do need a small EF02 partition (I make mine 4096 sectors), and then the rest of the disk as a BF02 partition. I wrote this up for an AWS AMI, but I use the exact same setup on my Debian desktops and servers - https://www.scotte.org/2016/12/ZFS-root-filesystem-on-AWS
This is true with any Linux-supported FS on most modern PCs since UEFI can only read VFAT partitions.