If I look at /usr/local/ispconfig/server/conf/nginx_vhost.conf.master I can see that http/2 is ready to be enabled. Code: listen <tmpl_var name='ip_address'>:<tmpl_var name='https_port'> ssl{tmpl_if name='enable_http2' op='==' value='y'} http2{/tmpl_if} So what are you supposed to do in the panel to enable it?
Enable SPDY/HTTP2 in the server-settngs (ssl) and make sure, that your webserver supports http2. iirc, you can enable this on a server that does not support http2 - http2 will removed from the vhost-config.