Replace GRUB with systemd-boot in Debian

If systemd-boot is already present in the EFI partition, create /boot/loader/entries/debian.conf.

title Debian  # Entry title
linux /vmlinuz-5.10.0-8-amd64
initrd /intel-ucode.img
initrd /initrd.img-5.10.0-8-amd64
options root=/dev/<system device> rw acpi_rev_override=1

Also, you need to create a /boot/loader/loader.conf file:

default Debian  # Default entry
timeout 10
console-mode max  # Console screen size.
editor no  # Disable entry edition
auto-entries 1
auto-firmware

After that you’ll need to do run # initramfs -uv to apply modification regarding the kernel on the vmlinuz and initrd file.
Voilà!