good news. Tesla lied and 2019.16.x.x software would work mostly fine after they stop vpn service. this is because the real tideover to "hermes" happened in 2019.8.x, so the only problem you will experience is if they stop openvpn service, your car will keep tryign to reconnect wich would increase the logging and add some cpu load, but at least the functionality should be fine I believe.
You can always turn to your friendly rooting community to help you stop vpn service from draining whateevr resources once the server side dsappears.
Not knowing the difference between vpn service and “hermes”, what are you saying will occur if one fails to update to at least 2019.40.2.3? Are you saying support for web sockets is already built into 2019.16.x.x and later, and everything will continue to operate as it does today?