x-powered-by is given by the module, for instance php fastcgi module offers to set expose_php=Offin php.ini
expose_php=Off
proxy_hide_header X-Powered-By;
fastcgi_hide_header X-Powered-By or proxy_hide_header X-Powered-By on http/server context
fastcgi_hide_header X-Powered-By
proxy_hide_header X-Powered-By