To become the Naib of a sietch a Fremen they must call out the current Naib via the tahaddi challenge and kill them in mortal combat.
Muad'Dib refused to kill Stilgar, Naib of his steich, when asked by the Fremen to become their leader.
Paul said that it was a waste of their best leaders and instead named himself as the Mahdi and Naib of all Naibs.
Was the tahaddi challenge removed as a requirement to be Naib for everyone? Or was Paul a singular exception to the rule? If it was removed or outlawed, was it ever reinstated?