[blessedwhiskers]

I had a Honeywell smart thermostat that worked entirely locally with Apple's HomeKit. I haven't used Home Assistant but a cursory Google search implies this would work. My recollection is it was a Honeywell T5. Notably it did sometimes fall off the WiFi for brief periods of time, so I couldn't give the product itself a ringing endorsement.

I usually specifically look for smart devices like this because I dislike the idea of them phoning home for "analytics" or "to enhance my marketing experience" and just firewall them off.

I'm not sure if it's explicitly required, but I've yet to encounter a HomeKit device that didn't work when prohibited from talking to anything outside the LAN.