Will iPhone XS work with Honeywell thermostat on the home app?

Which Honeywell thermostat? I have the T9 and it is not HomeKit enabled. Based on what I've seen online, Honeywell seems to be unenthusiastic about HomeKit compatibility. However, the Honeywell app works well on my iPhone 7 and on my Watch 4. The integration with Amazon's Alexa is disappointingly so-so.

I think it depends on that Honeywell model specs. I would contact Honeywell and ask them about the compatibility with your XS, running iOS 13 or later.

Regards

Giulio