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.

Y.Kohno

This is a flexible search form widget. 
You can easily generate a multi condition search form and display anywhere as widget.
複数のカスタムフィールド、投稿タイプ、カテゴリー、タグでの検索が可能な複合検索フォームを簡単作成できるプラグイン。ウィジェットで好きな場所に設置可能。
Currently supports 
– Custom Fields 
– Taxonomies 
– Categories 
– Post tags 
– Post Type

WP Meta Search

Search form for searching through custom fields, post types, categories, tags, and free text.

WP Categories Metabox Extended vs WP Meta Search