>To be clear, systemd-boot doesn't replace GRUB, in that systemd-boot can only boot other EFI binaries, so it still requires the kernel to be compiled as a UKI. A GRUB setup with a regular vmlinuz + separate initramfs in root partition (or boot partition that's not the ESP) can't be replaced with systemd-boot directly. You first need to switch to a UKI-in-ESP setup.

That's wrong, my laptop right now uses systemd-boot with a vmlinuz and an initramfs, no UKI. See a configuration example here: https://wiki.archlinux.org/title/Systemd-boot#Adding_loaders