search_users_by_meta.php

class Custom_REST_Users_Controller extends WP_REST_Users_Controller {

    /**
     * Registers the routes for the controller.
     */
    public function register_routes() {
        parent::register_routes();

        add_filter( 'rest_user_query', array( $this, 'filter_user_query_by_meta_fields' ), 10, 2 );
    }

    /**
     * Filter user query by meta fields.
     *
     * @param array $args The query arguments.
     * @param WP_REST_Request $request The current request object.
     * @return array The filtered query arguments.
     */
    public function filter_user_query_by_meta_fields( $args, $request ) {
        $meta_query = array();

        // Get the meta fields to search for.
        $meta_fields = $request->get_param( 'meta_fields' );

        if ( ! empty( $meta_fields ) ) {
            foreach ( $meta_fields as $meta_field ) {
                $meta_query[] = array(
                    'key'     => $meta_field['key'],
                    'value'   => $meta_field['value'],
                    'compare' => $meta_field['compare'],
                );
            }
        }

        if ( ! empty( $meta_query ) ) {
            $args['meta_query'] = $meta_query;
        }

        return $args;
    }
}