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.
You don't need to ban any of the 9's technically. Off the top of my head about half are SOS/SOD stuff. Shapchange has a HD limit that makes the really scary stuff inaccessible till epics, The rest are various forms of summons or direct damage, (lol irrelevant) and all either have a creature call in type/limit that keeps the CR inflation down, or are subject to DM fiat on what comes through, (which with a non-idiot DM amounts to the same thing as a CR limit). The same DM fiat applies to Wish/Miracle technically, those too are just such annoying spells that it's far easier to ban them than to try and deal with them via fiat. Astral Projection just puts an ethereal copy of you on the Astral Plane so unless the campaign takes place entirely there it's virtually worthless since as soon as you need to actually do anything in the material plane you have to end it get up, go off and actually do something. It would probably be awesome to make permanent on a kingdoms messengers, or create a circle version of it for the same purpose. But as anything related to a campaign on any other plane it's nigh worthless unless i missed an exploit or something.

The only way for 9's to break things to the point of beating stuff 11 levels up is an idiot DM that either doesn't employ DM fiat on the stuff he can to claw their power down, or that allows non-core stuff that would either give spells that are powerful enough for the job without innately allowing DM fiat, or non-core stuff that allows the effects of existing 9's to be pushed to levels way above what core allows.

Like i said, i'm not saying banning 9's is a bad call, because they break CR appropriate encounters quite firmly. But the point they hit the wall is a lot lower if an idiot isn't sitting in the DM's chair.