Maria Daniel Deepak

WP Categories Metabox Extended extends the Category metabox on your WordPress editor. Once installed the Category metabox gets a search bar with which you search Categories. Also, this is only seen on “Add New Post” screen in the WordPress Admin dashboard.
From <a href="https://wordpress.org/support/wordpress-version/version-5-0/" rel="ugc">WordPress 5.0</a>, the search bar is included with the Categories metabox. In case you choose to use the Classic editor by using plugins like <a href="https://wordpress.org/plugins/disable-gutenberg/" rel="ugc">Disable Gutenberg</a>, then WP Categories Metabox Extended plugin will continue to extend the Categories metabox for the default “Post” post type.

WP Categories Metabox Extended

WP Categories Metabox Extended plugin allows you to search and assign Categories to your post in your WordPress editor.

wokamoto

You can choose (child) categories and see the posts belong to the chosen category with AJAX.
When the category you choose from the list box has child categories, the child categories will be shown in a new list box. 
Clicking the “Search” button, the list of posts belong to the bottom category which you choose will be displayed under the “Search” button.

Simple Category Search

You can choose (child) categories and see the posts belong to the chosen category with AJAX.

WP Categories Metabox Extended vs Simple Category Search