lj/sf is an alias for lj/smooth/linear that was added for backward compatibility.
you probably have looked at an outdated left-over documentation file on the old LAMMPS homepage.
Since we moved the homepage recently to a different domain and a different server, we weeded out all files that were left behind and had not been cleaned up.
the proper documentation is here: https://docs.lammps.org/pair_lj_smooth_linear.html
axel