You thought Google had a plan for your Nest Protect with the changeover from Nest accounts, to Google Nest accounts? So did I.
Unfortunately after a very long chat with both Google and Nest support people, it's evident that they don't have a plan to keep this working and their support people are seriously uninformed about the ramifications of such a change.
However, if you are using @dan.t 's fantastic Maker API version of the Homebridge app for Hubitat like I am, you will know how good the performance is. Eariler versions of the Hubitat app for Homebridge simply were too temperamental, so I had previously limited the plugins to HE only.
Now, I'm finding I can start adding plugins back to Homebridge, with no degradation in performance. With that in mind, I set out to find the best plug-in to bring Nest products into Homebridge. To my delight, I found that the homebridge-nest plugin, not only handles the task very well, but it also no longer uses the "Works with Nest" API. So come August 31, 2019, the homebridge-nest plugin will be completely unaffected by the changes google is making.
The plugin supports 2-Factor authentication from your Nest account, and even supports access tokens if you don't want to enter your Nest username and password in the config.json file. Once exposed to HomeKit, I simply used HomeKit automations to link the Nest Protects Smoke and CO emergencies to virtual switches. When the virtual switches turn on, my RM rules for each event will run actions to turn on all the lights, change capable lights to red, and give repeating instructions via Alexa announcements at full volume.
Not for everyone, but those with Nest Protects and HomeKit automation capability should find this a valuable addition. The connection from Homebridge to the Nest Protect is still cloud, but all operations to turn on lights and trigger actions based on the Nest Protect alarm states can now be fully local, unlike the previous method using Nest cloud instance -> IFTTT cloud instance -> Hubitat cloud instance, before any local actions could be executed.