nginx transparently proxies using http2 to client and http1 to server. In a standard setup at any scale with a load balancer it doesn't matter that Rails doesn't support all of HTTP2. The client still gets all the benefits of header compression etc.
What is a standard setup in 2019, there is none? There is also no standard in using nginx as front facing entity.
In my prior comment I was friendly and gave an example why some might not have nginx in their stack and BTW the discussion is about Rails lacking http2 and you try to convince me that this is expected and right.
This is pure ignorance paired with incompetence, maybe in your small world things work this way. Let's just stop here and go back to your legacy stack.
This is why nobody is rushing out to rewrite Rack to support HTTP2. Ruby web apps already benefit from almost all of the advantages of HTTP2.