@ningji that's been there for a long time now. You just need to have one of the search plugins installed and activated.
nodebb-plugin-dbsearchgets installed by default. Activate the plugin and restart NodeBB to see the search icon.
@yariplus FYI... plugins themselves don't add that search menu. NodeBB checks for any active plugins listening for the hook
filter:search.queryto decide whether to show or hide the search icon.
Thanks very much, will give it a try later !