How to tackle the downtime for maintenance of nginx

See https://docs.nginx.com/nginx/admin-guide/load-balancer/http-health-check/

http {
    #...    match server_ok {
        status 200-399;
        body   !~ "maintenance mode";
    }
    server {
        #...        location / {
            proxy_pass   http://backend;
            health_check match=server_ok;
        }
    }
}

Here the health check is passed if the status code of the response is in the range 200–399, and its body does not contain the string maintenance mode.