If I'm gonna invest in a heat pump for the long term, I don't want it committed to any software technology that may be obsolete in 10 years. I want it to use something so dead stupid simple that anything in 50 years can still operate it without a lot of work.

The simplest example I can think of is MIME (https://en.wikipedia.org/wiki/MIME). It can be embedded in basically any transport type. It can encode anything you want (binary, UTF8, etc). It is very old, stable and well-tested. It is fairly trivial to implement a limited instruction set of.

Transport security, authentication, authorization, etc, are nice, but not nearly as important to me as a future-proof interface to my heat pump. Give me the option to enable an admin port and just run a single unsecured connection with a basic dumb text protocol, and I can hack together a client in 30 minutes in any programming language. It's not fancy but it will work forever.