That's a Nest issue, not an apple one. Google neutered their own API as of 2019, then did even more shenanigans after that. It's a google product, assume it will be killed the moment whoever is actively working it sees a new shiny.

The new nest thermostats can be controlled by Apple Home fwiw

(They support Matter protocol)

I find it frustrating that I can’t drive HomeKit externally. I have a large home ecosystem, something in the order of 100 devices, from lights to outlets to motion sensors to security and locks to the garage door, some native and some bridged via HomeBridge. I have a bunch of scenes and automations.

But I have, for example, a couple of Brilliant smart wall panels (one gang sized little panels with lighting and other controls) that I have to duplicate because despite the Brilliant being able to expose devices as a bridge to HomeKit I can’t say “Run this Scene” because Apple doesn’t allow external integrations to do so.