> How exactly would this treat e.g.

    yt $URL/somefile

> Would the variable still be expanded? Would an asterisk glob be?

No. Pure should mean don't do anything from here on, just pass the arguments to the function.

> If I _did_ want it to expand something even though the function is declared "pure", how would that work?

If expanding should be possible in a pure context, then maybe by using a known syntax like:

    yt ($URL)/somefile

Eshell expands Lisp code when using (lisp-code-here) which is kind of elegant. So when in a pure context, you would have to switch to expansion-mode by using curly braces.