Was it ever actually enforced? I brought back the "God" creature from TFF in my Q Continuum trilogy as far back as 1998, and referenced Sybok in Child of Two Worlds. Nobody at CBS/Paramount or Pocket Books ever objected.
(I suppose it's possible that TFF was off-limits back when Richard Arnold was approving the novels on Roddenberry's behalf, but that was before my time. I can testify that it wasn't enforced policy after Roddenberry passed away and Arnold was let go back around 1991, about thirty-five years ago.)