Amaury Balmer

WordPress 3.1 and up allow for reasonably simple custom taxonomy, this plugin makes it even simpler, removing the need for you to write any code.
Add support for taxonomy with or without hierarchical. Choose the selector type for write post.
Allow to add taxonomy to any objects registered on your installation. This plugin can be coupled with the plugin ACF to a new term meta database that allow to create on fly any field need for the taxonomy. For example, for a “Number” taxonomy, you can add fields : number ID, date of parution, screenshot, etc !
This plugin is developped on WordPress 3.5, with the constant WP_DEBUG to TRUE.
This plugin allows you to add taxonomy just by giving them a name and somes options in the backend. It then creates the taxonomy for you, takes care of the URL rewrites, provides a widget you can use to display a “taxonomy cloud” or a list of all the stuff in there, and it allows you to show the taxonomy contents at the end of posts and excerpts as well. 
To boot, it comes with a set of template tags for those who don’t mind adapting their own theme.
For full info go the <a href="https://github.com/herewithme/simple-taxonomy" rel="nofollow ugc">Simple Taxonomy</a> page.

Simple Taxonomy

WordPress 3.1 and up allow for reasonably simple custom taxonomy, this plugin makes it even simpler, removing the need for you to write any code.

nwjames

Supports adding one or more taxonomies (either hierarchical or tag) to any objects registered on your installation.
This plugin started as a functional conversion from <a href="https://wordpress.org/plugins/simple-taxonomy/" rel="ugc">Simple Taxonomy</a> (now closed) and developed on WordPress 5.1-6.9 with PHP 7.2-8.4.
This plugin allows you to add a taxonomy just by giving them a name and some options in the backend. It then creates the taxonomy for you and takes care of the URL rewrites.
It provides a widget that you can use to display a “taxonomy cloud” or a list of all the terms; it allows you to show the taxonomy contents at the end of posts and excerpts as well. To increase flexibility, a shortcode and block has been provided to output these terms wherever desired.
You can also export the Taxonomy definition to include it directly in your own code.
You can also create terms easily by typing them into a list; or by copying them from an existing taxonomy.
A tool has been provided to support changing the taxonomy slug. Any terms and their usages will also be linked to the renamed slug.
For admin screens displaying multiple taxonomies it is possible to define their display column order.
A tool is provided to merge a number of terms within a taxonomy into a single one. All usages of the selected terms are changed to the merged one.
Options are provided to add a selection dropdown in the admin list and to define minimum and maximum required term counts using posts of selected statuses (and not only “published”). [Term counts with WP 5.7+ requires version 1.3+ of this plugin.] These capabilities are available for any taxonomy whether defined using this taxonomy or elsewhere.
For full information go the <a href="https://github.com/NeilWJames/simple-taxonomy-refreshed/blob/master/docs/readme.md" rel="nofollow ugc">Simple Taxonomy Refreshed</a> page.
When using the admin screen, additional information is available in the help pulldown area.
<h3>Migration Notice</h3>
<h4>From Simple Taxonomy</h4>
It is a drop-in replacement for <a href="https://wordpress.org/plugins/simple-taxonomy" rel="ugc">Simple Taxonomy</a> – using the same options table entry.
If this is installed, deactivate it first.
However since this plugin uses the Simple Taxonomy options data to save setting it up again completely if you wish to revert, before deactivating you can use the Simple Taxonomy export function to take a copy of your data.
NB. The Export/Import functions are not compatible between plugins. So you need to use the file made with its version of the plugin.
To have the Taxonomy metaboxes available in the Block Editor, ensure that “show_in_rest” has been set to true.
When migrating and before an update to the existing parameters, the taxonomy will treat “show_in_rest” as true. If not wanted, set to false.

Simple Taxonomy Refreshed

This plugin provides a no-code facility to manage your taxonomies - either by defining your own or by adding additional function to existing ones.

Simple Taxonomy vs Simple Taxonomy Refreshed