I can't even imagine someone makeing a 12+ level adventure and having to "account for" something cool the fighter can do. On the other hand EVERY 12+ level adventure has to be built thinking about 3rd-6th level spells
Oftentimes you don't have to; a sufficiently motivated martial will find things to do in an encounter that requires sufficiently divided attention.
Wizards get a lot of hype but they can't actually do everything all at once, nor can any of the other casters.
And that's all before you start looking at encounter designs that focus on needing more than one
player, not just a specific set of classes. The classic "I stand here you stand over there" puzzle is the simplest example of how encounter design can help equalize even extremely disparate characters.
And before anyone tries to jump down my throat, here is the disclaimer stating that:
1. Yes, a Caster could probably solve that deliberately simple example on their own. Not only is the example
not meant to be anything more than it is, but this also just points at magic as being badly designed, as argued previously.
2. No, stating that encounter design can help address class disparities
does not mean there isn't still a problem even if you do it.
3. No, I am
not stating there is no problem with class disparities by expressing a solution that doesn't address it directly.