"#some_combo:has(option:selected[value=..]) + .." seems like a reasonable way to conditionally target something to me, is it terribly worse than some other way?
Yeah, it's fragile and will easily lead to bugs when someone changes the markup without realizing it's going to break some crazy selector in another part of the code.
It would make a lot more sense to just add a class to the element you're trying to select.
Look, if you need this for some one off thing and you've determined it's the best way to do it in this special case, it's not hard to create a function that will find the element you want.
It's not a good argument for using jQuery IMO, because if you're doing this regularly there's probably a better way to do it.
But coming soon™ you will be able to do even this with `document.querySelector`.
Edit: I didn't pay close attention to your second example. I was speaking mostly related to the a:has example before. Your second example seems to be something that would be desired more in CSS than JS, and I don't think it's unreasonable to do that in CSS. If you need to do it in JS you can workaround browser limitations just fine by writing more than one line of code to do the selection and test the condition.