@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-dbsearch gets 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.query to decide whether to show or hide the search icon.