Maintenance Mode for Non-Admins

<?php
add_action( 'template_redirect', 'scseed_maintenance_mode', 1 );
function scseed_maintenance_mode() {
    // Allow administrators, CLI processes, and WP-Cron through.
    if ( current_user_can( 'manage_options' ) ) {
        return;
    }
    if ( defined( 'WP_CLI' ) && WP_CLI ) {
        return;
    }
    if ( defined( 'DOING_CRON' ) && DOING_CRON ) {
        return;
    }

    // 503 tells search engines the site is temporarily unavailable;
    // Retry-After suggests when to check back.
    status_header( 503 );
    header( 'Retry-After: 3600' );
    nocache_headers();

    $site_name = get_bloginfo( 'name' );

    wp_die(
        '<div style="font-family:sans-serif;text-align:center;padding:60px 20px;">'
        . '<h1>' . esc_html__( 'Briefly unavailable for scheduled maintenance.', 'scseed' ) . '</h1>'
        . '<p>' . esc_html__( 'Check back in a moment.', 'scseed' ) . '</p>'
        . '<p style="color:#888;font-size:0.9em;">' . esc_html( $site_name ) . '</p>'
        . '</div>',
        esc_html__( 'Maintenance Mode', 'scseed' ),
        array( 'response' => 503 )
    );
}