Google announced the end of support for early Nest Thermostats in a support document earlier this year that largely flew under the radar. As of October 25, first and second generation units released in 2011 and 2012, respectively, will be unpaired and removed from the Google Nest or Google Home app.
Users will no longer be able to control their thermostats remotely via their smartphone, receive notifications, or change settings from a mobile device. End-of-support also disables third-party assistants and other cloud-based features including multi-device Eco mode and Nest Protect connectivity.
There is an open source project to replace the innards:
https://sett.homes/
While an interesting idea, that project wants what amounts to the cost of a new thermostat in exchange for a kickstarter project. Might as well just buy a new thermostat.