Get all taxonomies and terms in WordPress

$args = array(
    'public'   => true, // only load publicly visible taxonomies
    'publicly_queryable' => true, // another check for public taxonomies
    '_builtin' => false, // excludes WordPress's built-in taxonomies (like category and post_tag)
);

$taxonomies = get_taxonomies($args, 'objects');

$taxonomy_terms = array();

foreach ($taxonomies as $taxonomy) {
    $terms = get_terms(array(
        'taxonomy'   => $taxonomy->name,
        'hide_empty' => false,
    ));

    if (!empty($terms) && !is_wp_error($terms)) {
        $taxonomy_terms[$taxonomy->name] = wp_list_pluck($terms, 'name');
    } else {
        $taxonomy_terms[$taxonomy->name] = array();
    }
}

return $taxonomy_terms;