optgroup
There are 4 repositories under optgroup topic.
X13454/bootstrap-selectsplitter
Transforms a <select> containing one or more <optgroup> into two chained <select>
lomars/woocommerce-select-form-field-with-options-group
Add select field with optgroup new field type to WooCommerce form fields
CarlesRever/dropdown-submenu-jquery
Turn the native HTML select into a fancy dropdown with floating submenus
mikeludemann/selectionMethods
Some methods and examples for updating and changing selects with options on another selects and these options