I recently reinstalled my system and struggled to find the right configuration to use systemd-boot with Btrfs as a root subvolume so here it is in case it helps someone.

# /boot/loader/loader.conf
default @saved
editor yes
timeout 3
# /boot/loader/entries/arch.conf
title   Arch Linux
linux   vmlinuz-linux
initrd  initramfs-linux.img
options rd.luks.name=<UUID>=root root=/dev/mapper/root rootflags=subvol=@

Replace <UUID> with the UUID of the physical disk partition. The rootflags=subvol=@ part is important and should not contain any slashes, i.e. rootflags=subvol=/@ won’t work. Don’t ask me how much time I wasted on that…