I know Lennox S30, Carrier Cor, Bryant Housewise "thermostats" are HomeKit compatible. I quoted the word thermostats because I know the Lennox S30 is actually a controller that communicates with the equipment rather then a simple thermostat (switch controlled by temp). Depending on the thermostat, integrating functions into HomeKit aren't easily possible, for example Rheem/Ruuds EcoNet communicating controller can also control a Rheem water heater temp functions like that are difficult to integrate.
Assuming a thermostat isn't a proprietary controller for that manufacturers HVAC equipment you or your HVAC contractor can install a HomeKit thermostat at time of purchase or later down the road.