That sounds like 'some from column A, some from column B'.
On the one hand, yeah, the server should be able to negotiate the version of HTTP, or degrade to the long-standing HTTP/1.1 if the client doesn't handle HTTP/2, or at the very least give a meaningful error message that says, "Whoops, this only works with HTTP/2, sorry". That's a , but not the big one here.
A brand-new version of iOS not supporting HTTP/2? Now that is a major .