This is the much easier way to deal with a full /boot:
nix-collect-garbage --delete-older-than 14d # or some other value
Alternatively just remove a number of old generations from /nix/var/nix/profiles:
rm /nix/var/nix/profiles/system-some-old-generations # followed by
nix-store --delete /nix/store/whatever-the-various-old-generations-point-at
Then to clean up the old kernels/initrds:
/run/current-system/bin/switch-to-configuration boot