Does http4k support HTTPS server-side?

You'd do this by configuring a backend server adapter class. As HTTPS is generally terminated at a higher level, we don't build support for it into the libraries due to the complexity and variability of the requirements. However, you can easily implement your own ServerConfig class (using the library versions as a base).

Is this possible with Netty, or only with Jetty?

I'm not an expert in the details of configuring Netty TBH - you'd have to take a look at the docs. This is precisely the reason that we don't get involved in the maze of server configuration and just provide the ServerConfig interface to plug into 🙂