WordPress: Add support to `get_terms()` to limit results to Terms used by Post Type.

functions.php

<?php
/**
 * Terms_clauses
 *
 * Filter the terms clauses.
 *
 * @param $clauses array
 * @param $taxonomy string
 * @param $args array
 * @return array
**/
function terms_clauses( $clauses, $taxonomy, $args ) {
  
	global $wpdb;

	if ( array_key_exists( 'post_type', $args ) ) {

		$post_types = $args['post_type'];

		if ( is_array( $args['post_type'] ) ) {

			$post_types = implode( "','", $args['post_type'] );

		}

		$clauses['join']  .= " INNER JOIN $wpdb->term_relationships AS r ON r.term_taxonomy_id = tt.term_taxonomy_id INNER JOIN $wpdb->posts AS p ON p.ID = r.object_id";
		$clauses['where'] .= " AND p.post_type IN ('" . esc_sql( $post_types ) . "') GROUP BY t.term_id";
	}

	return $clauses;

}
add_filter( 'terms_clauses', 'terms_clauses', 10, 3 );

Usage

$term = array(
	'post_type' => 'POST_TYPE',
);

$terms = get_terms( 'post_tag', $args );

Example output

array(1) {
  [0] => object(WP_Term) {
    ["term_id"]          => int()
    ["name"]             => string()
    ["slug"]             => string()
    ["term_group"]       => int()
    ["term_taxonomy_id"] => int()
    ["taxonomy"]         => string()
    ["description"]      => string()
    ["parent"]           => int()
    ["count"]            => int()
    ["filter"]           => string()
    ["term_order"]       => string()
  }
}