Source

File: wp-includes/class-wp-xmlrpc-server.php

public function get_term_custom_fields( $term_id ) {
		$term_id = (int) $term_id;

		$custom_fields = array();

		foreach ( (array) has_term_meta( $term_id ) as $meta ) {

			if ( ! current_user_can( 'edit_term_meta', $term_id ) ) {
				continue;
			}

			$custom_fields[] = array(
				'id'    => $meta['meta_id'],
				'key'   => $meta['meta_key'],
				'value' => $meta['meta_value'],
			);
		}

		return $custom_fields;
	}