Order WordPress Terms by Term Meta: https://wordpress.org/plugins/advanced-term-fields/

filter-terms-clauses.php

<?php

/**
 * Filter the terms query SQL clauses.
 *
 * @see 'terms_clauses' filter in get_terms() wp-includes/taxonomy.php
 *
 * @since 0.1.0
 *
 * @todo add filter for $allowed_orderby_keys
 *
 * @param array $pieces     Terms query SQL clauses.
 * @param array $taxonomies An array of taxonomies.
 * @param array $args       An array of terms query arguments.
 * 
 * @return array $pieces The filtered SQL clauses
 */
function filter_terms_clauses( $pieces = array(), $taxonomies = array(), $args = array() )
{
    global $wpdb;

    /**
     * If we're not ordering by any of the allowed keys, return
     */
    $orderby = ( ! empty( $args['orderby'] ) ) ? $args['orderby'] : '' ;
    $allowed_orderby_keys = array( 'meta_value', 'meta_value_num' );

    if ( ! in_array( $orderby, $allowed_orderby_keys, true ) ) {
        return $pieces ;
    }

    // Bail if there's no meta query
    if( empty( $args['meta_query'] ) ) {
        return $pieces ;
    }

    switch ( $args[ 'orderby' ] ) {
        case 'meta_value' :
            $pieces ['orderby'] = "ORDER BY {$wpdb->termmeta}.meta_value";
            break;
        case 'meta_value_num':
            $pieces ['orderby'] = "ORDER BY {$wpdb->termmeta}.meta_value+0";
            break;
    }

    return $pieces ;
}

add_filter( 'terms_clauses', array($this, 'filter_terms_clauses'), 10, 3 );



/**
 * Call your terms like so:
 * 
 * Replace "my_meta_key" with  the name of the requested meta key
 */ 
$args = array(
    'hide_empty' => false,
    'orderby' => 'meta_value',
    'order' => 'ASC',	
    'meta_query' => array(
        'relation' => 'OR',
        array( 
            'key'=>'my_meta_key',
            'compare' => 'EXISTS'           
        ),
        array( 
            'key'=>'my_meta_key',
            'compare' => 'NOT EXISTS'           
        )
    ),
);

get_terms($taxonomy, $args);