I did this

curl -x http://proxyname:8080 -v --http2-prior-knowledge http://httpbin.org/get

I expected the following

I expected curl to proxy the request but instead it says Ignoring HTTP/2 prior knowledge due to proxy and shows error curl: (1) Received HTTP/0.9 when not allowed. The VM snapshot is from 2019 so at some point around then it used to work.

curl/libcurl version

master

operating system

server {
    listen 8080 http2;
    include snippets/snakeoil.conf;

    allow  [REMOVED];
    deny   all;

    location / {
        resolver 8.8.8.8;
        proxy_pass http://$http_host$uri$is_args$args;
        proxy_set_header Proxy-Connection "";
    }

    error_page   500 502 503 504  /50x.html;
    location = /50x.html {
        root   html;
    }
}