There is a class action "mass arbitration" against Google for this:
https://www.classaction.org/nest-thermostat-support-arbitration
Additionally, the Fulu Foundation has a bounty reward out for anyone who is able to get these working with something like Home Assistant.
The pot is currently at $12,856.00 https://bounties.fulu.org/bounties/nest-learning-thermostat-gen-1-2
In the U.S., since doing so would circumvent measures put in place on these devices, publishing how to do this would go against sec. 1201 of the DMCA. This has a risk of a maximum sentence of 3-5 years in a Federal Prison. You can still privately show the Fulu Foundation how it is done, and they will be able to use this information to help their case in their attempt to reform this law.
If you live in the U.S., you can also help by letting your representatives know about this. Here's an ActionNetwork page that Fulu set up so that you can easily do so: https://actionnetwork.org/letters/right-to-repair-reform-section-1201-of-the-dmca