OptGroup Taxonomy Select

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.