This is one of the awesome things about Plug, in the sense that if your library or what have you targets Plug it'll work reasonably well (if not entirely seamlessly) in a Phoenix app, a Sugar app, a pure-Plug app, a $SomeOtherFramework app, or what have you.

Plug happens to be somewhere in between the two outcomes you've presented, of course; its development happens very closely to that of Elixir (and it's arguably a sub-project thereof), but it ain't a core part of the language itself (last I checked; maybe this has changed in recent years).