Check if maintenance mode is enabled.

Checks for a file in the WordPress root directory named “.maintenance”.
This file will contain the variable $upgrading, set to the time the file
was created. If the file was created less than 10 minutes ago, WordPress
is in maintenance mode.

function wp_is_maintenance_mode() {
	global $upgrading;

	if ( ! file_exists( ABSPATH . '.maintenance' ) || wp_installing() ) {
		return false;
	}

	require ABSPATH . '.maintenance';
	// If the $upgrading timestamp is older than 10 minutes, consider maintenance over.
	if ( ( time() - $upgrading ) >= 10 * MINUTE_IN_SECONDS ) {
		return false;
	}

	/**
	 * Filters whether to enable maintenance mode.
	 *
	 * This filter runs before it can be used by plugins. It is designed for
	 * non-web runtimes. If this filter returns true, maintenance mode will be
	 * active and the request will end. If false, the request will be allowed to
	 * continue processing even if maintenance mode should be active.
	 *
	 * @since 4.6.0
	 *
	 * @param bool $enable_checks Whether to enable maintenance mode. Default true.
	 * @param int  $upgrading     The timestamp set in the .maintenance file.
	 */
	if ( ! apply_filters( 'enable_maintenance_mode', true, $upgrading ) ) {
		return false;
	}

	return true;
}