Is Slann mage priest affected by dreaded 13th? Slann is in fact an Infantry model but has an special rule that ignores effects done against infantry models in spells, special rules and so on. Should it be removed? Or survive the effects?
Unless the dread 13th spell specifically says it only affects models on foot then the Slann is still affected. The rule only allows it to ignore spells, magic items and special rules that affect models on foot.
Dreaded 13th removes 4d6 infantry models from targeted unit. If the inmunity is against model on foot rules, what kind of rules are affecting models on foot? If i am not wrong all this kind of special rules states infanty type of troop models, not models on foot. -Killing blow -Stomp - Other rules i can't remember. The only example I find is Transformation of Kadon. (and only to allow a MI models to use it) So it seems to be a nerf to the slann as no benefits.
Pretty much as you say, there's no real benefit for the Slann to have to rule. I think there use to be a spell from the Lore of Nurgle that only affected models on foot but that was a while ago. I believe the rule is a hand over from the last book were the Slann had levitation (due to 7th editions LOS rules). For fluff reason it still makes sense though, as Slann can't touch the ground else they'd ground their magical powers.
Or pit of shades maybe? How can he fall into a pit if he is floating in the air? Sadly these things happen... Insert appropriate fluff and or imagination and roll with the punches.
I think the point of the palenquin is to hover a certain distance over the ground. Kind of like repelling magnets. As the spells you mentioned effectively create a hole below the slann; he would still fall into it. The on foot rule also effects some magic items, for example you can't give the Slann a magic carpet it cause him to fly.