Register Custom Variable for Rank Math (state_slug)

rank-math-custom-variable.php

<?php
add_action( 'rank_math/vars/register_extra_replacements', function() {
    rank_math_register_var_replacement(
        'state_slug',
        [
            'name'        => esc_html__( 'State Slug', 'rank-math' ),
            'description' => esc_html__( 'Displays the slug of the States custom taxonomy in uppercase.', 'rank-math' ),
            'variable'    => 'state_slug',
            'example'     => 'VA',
        ],
        'get_state_slug_var'
    );
});

function get_state_slug_var() {
    global $post;
    $terms = get_the_terms($post->ID, 'states');
    $state_slug = '';
    if ($terms && !is_wp_error($terms)) {
        $state_slug = strtoupper($terms[0]->slug);
    }
    return $state_slug;
}