Triweb Digital

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.

Taxonomy Admin Search

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

HelgaTheViking

Replace the default taxonomy boxes with a custom metabox that uses radio buttons… effectively limiting each post to a single term in that taxonomy.
A plugin options page allows the user can select which taxonomies she’d like to switch to using the custom radio-button style metabox.
Originally based on the the <a href="https://github.com/stephenh1988/Radio-Buttons-for-Taxonomies" rel="nofollow ugc">class by Stephen Harris</a>
<h4>Support</h4>
Support is handled in the <a href="https://wordpress.org/support/plugin/radio-buttons-for-taxonomies" rel="ugc">WordPress forums</a>. Please note that support is limited and does not cover any custom implementation of the plugin. Before posting a question, read the <a href="https://wordpress.org/plugins/radio-buttons-for-taxonomies/#faq" rel="ugc">FAQ</a> and confirm that the problem still exists with a default theme and with all other plugins disabled.
Please report any bugs, errors, warnings, code problems to <a href="https://github.com/helgatheviking/Radio-Buttons-for-Taxonomies/issues" rel="nofollow ugc">Github</a>

Radio Buttons for Taxonomies

Thank you so much for this plugin, so easy to use and makes creating a single category situations much easier.

Oh this is a life saver!

JUST WORKS

Many thanks to the author for this great plugin. Does one things and does it very well. Works with classic editor and block editor.

5 stars is not enough. Works with Gutenberg

We've been using this plugin for longer than we can remember, never had any issues whatsoever, highly recommended!

Excellent

Replace the default taxonomy boxes with a custom metabox that uses radio buttons... effectively limiting each post to a single term in that taxonomy.

Taxonomy Admin Search vs Radio Buttons for Taxonomies