OptGroup Taxonomy Select
This module improves taxonomy term selection by displaying terms in a hierarchical, grouped dropdown list. It makes long lists of taxonomy terms easier to navigate for users.
This module provides a widget dedicated for taxonomy term reference field. It displays the terms list with an OptGroup format.All the nested level terms are also grouped under the main parent term.
The <optgroup> tag is used to group related options in a <select> element (drop-down list).
If you have a long list of options, groups of related options are easier to handle for a user.
Reference:
https://www.w3schools.com/tags/tag_optgroup.asp
Installation:
Install with Composer: $ composer require 'drupal/optgroup_taxonomy_select'
Note:
Clear the Drupal cache after saving the module configuration settings.