HTTP/2: "http2" directive. The directive enables HTTP/2 in the current server. The previous way to enable HTTP/2 via "listen ... http2" is now deprecated. The new approach allows to share HTTP/2 and HTTP/0.9-1.1 on the same port. For SSL connections, HTTP/2 is now selected by ALPN callback based on whether the protocol is enabled in the virtual server chosen by SNI. This however only works since OpenSSL 1.0.2h, where ALPN callback is invoked after SNI callback. For older versions of OpenSSL, HTTP/2 is enabled based on the default virtual server configuration. For plain TCP connections, HTTP/2 is now auto-detected by HTTP/2 preface, if HTTP/2 is enabled in the default virtual server. If preface is not matched, HTTP/0.9-1.1 is assumed.

--- a/src/http/ngx_http_core_module.c
+++ b/src/http/ngx_http_core_module.c
@@ -4176,6 +4176,11 @@ ngx_http_core_listen(ngx_conf_t *cf, ngx
 
         if (ngx_strcmp(value[n].data, "http2") == 0) {
 #if (NGX_HTTP_V2)
+            ngx_conf_log_error(NGX_LOG_WARN, cf, 0,
+                               "the \"listen ... http2\" directive "
+                               "is deprecated, use "
+                               "the \"http2\" directive instead");
+
             lsopt.http2 = 1;
             continue;
 #else