This article describes how to disable Parallels RAS HTML5 Gateway functionality.
To do so, please go to RAS Console → Farm → Gateways, open properties of each gateway, go to HTML5 tab and uncheck the option Enable HTML5 Client. If all gateways inherit site default settings, you may toggle this option in "Site defaults" available on the same tab:
Users will receive error 404 Not Found RAS SecureClientGateway failed to connect to HTTP server(s) when accessing the old URL like https://yourdomain.com
If you'd like to redirect them to some other website instead of non-working HTML5 GW, please set its address in Default URL on the Web tab: