Uh, if you use a list of spells that's "Core only and none of the most powerful 9ths" then restrict to core feats and core creatures, then maybe a wizard would have trouble with CR+11 challenges. I'm not disagreeing there. Are you also banning astral projection? Because that would work too.

That's significantly different from "A wizard can't fight CR+11 challenges in the ELH." A Wizard with 9s has effective power to beat any challenge that gets its CR based on being a meatbag, which to be honest is a lot of them. Maybe most.

One of the reasons I banned wizards, and classes of similar power.