custom form list field add disabled options - create optgroups

Question

I have a working list based on an sql query. Now I would like to add some spacers (not selectable items).

When doing this manually in xml you can add the disabled attr to that item, but how can I do that when filling the list like so:

$cities = array(
    array('value' => 1, 'text' => 'New York'),
    array('value' => 2, 'text' => 'Chicago'),
    array('value' => 3, 'text' => 'San Francisco'),
);

I have tried to add 'disabled' => 'true' to the item array and some different versions of 'attribute' => 'disabled', but none of them worked so far.

EDIT: What it should look like:

<option value="" disabled>---Letters---</option>
<option value="x">X</option>
<option value="y">Y</option>
<option value="z">Z</option>
<option value="" disabled>---Numbers---</option>
<option value="1">1</option>
<option value="2">2</option>
...

In the rendered output, Letters and Numbers should just be a "divider" in the select list that is not selectable - "optgroups" are the right keyword, yes.

Edit 2: Okay I've got it now, partly, with the information from mickmackusa:

public function getOptions() {
    $type = $this->element['context'];
    $component = $this->element['component'] ? $this->element['component'] : 'com_content';
    $context = $component . '.' . $type;

    $jcFields  = FieldsHelper::getFields($context, '*', true); // get custom field names by article id

    $options = array();

    $options[] = JHTML::_('select.optgroup', '---Article---');
    $options[] = JHTML::_( 'select.option', 'nxItemId', '***Item ID***' );

    $options[] = JHTML::_('select.optgroup', '---Customfields---');
    foreach ($jcFields as $jcField){
        $options[] = JHTML::_( 'select.option', $jcField->name, $jcField->title );
    }
    
    return $options;
}

The second optgroup header (---Customfields---) will not be rendered.

Answer 1

My question could be solved by the comments of mickmackusa and Sharky. I have finally extended the JFormFieldGroupedList as follows:

defined('JPATH_PLATFORM') or die;
JFormHelper::loadFieldClass('groupedlist');
JLoader::register('FieldsHelper', JPATH_ADMINISTRATOR . '/components/com_fields/helpers/fields.php'); //load fields helper

class JFormFieldNxCustomfieldListKey extends JFormFieldGroupedList
{
    protected $type = 'nxCustomfieldListKey';

    public function getGroups() {

        $jcArticleFields  = FieldsHelper::getFields('com_content.article', '*', true); // get custom field names by article id
        $jcContactFields  = FieldsHelper::getFields('com_contact.contact', '*', true); // get custom field names by article id

        $groups = [];

        // Add Module Style Field
        $tmp = JText::_('MOD_NX_GROUP_BASIC');
        $groups[$tmp][] = JHtml::_('select.option', '', JText::_('MOD_NX_EXPOSER_DISABLED'));
        $groups[$tmp][] = JHtml::_('select.option', 'nxItemId', 'Item ID');


        // Create one new option object for each available style, grouped by templates
        $groups['Article Fields'] = [];
        $groups['Contact CustomFields'] = [];

        $grp = JText::_('MOD_NX_ARTICLE_FIELDS');
        foreach ($jcArticleFields as $jcField){
            $groups[$grp][] = JHtml::_('select.option', $jcField->name, $jcField->title);
        }

        $grp = JText::_('MOD_NX_CONTACT_FIELDS');
        foreach ($jcContactFields as $jcField){
            $groups[$grp][] = JHtml::_('select.option', $jcField->name, $jcField->title);
        }

        return $groups;
    }
}