If Nasus casts Wither on a champion with slow immmunity, that champion will also negate cripple.

Tested. The debuff won't even be placed on the target (yi, sejuani etc..), and if the immunity activates after Wither gets placed the debuff will get cleansed.

If the target however has 100% slow resist (which is different than slow immunity) only the slow is negated.