I don’t have experience with the Honeywell thermostats. Depending on the model, i believe you have the option to either use the HomeKit integration or the native Honeywell integration, if the thermostat supports their cloud service.
I have been using an Ecobee thermostat for a number of years. Unfortunately, as you noted, they’ve stoped registering new developer accounts, which are needed by the native integration for API access. There is still the option to use the HomeKit integration with the Ecobee thermostats.
https://community.home-assistant.io/t/how-to-migrate-from-ecobee-cloud-integration-to-local-homekit-controller/491319
-Brent