It seems you might’ve misunderstood my question (or I just phrased it wrongly).
I’m not asking what my site should use, more what my site should use to talk to nginx.
I might be misunderstanding how nginx works, but I thought that nginx parses all http requests, then picks the place to send them, then reencodes them for that place.
The docs state that HTTP/1 is used by default (which does seem a little weird because I believe every backend should support at least 1.1).