by Triweb Digital on WordPress.org
Adds an AJAX-powered search box to taxonomy metaboxes on the post edit screen when a taxonomy has more than 5 terms.

Search box above taxonomy metabox.
This plugin injects a small search box at the top of taxonomy metaboxes (the checkbox lists) on the post edit screen. It only appears for taxonomies that have more than 5 terms to avoid clutter.
Search queries are performed via AJAX and the checklist is filtered client-side for a quick, responsive experience.