Until recently, that is. I started noticing the battery plummeting during the day and when I checked usage, it was the Bluetooth radio. Sure enough, after a couple of days of this, it announced an update was ready. I checked the firewall, and it was still blocked, so... Worse, it had downloaded the update over LTE. I factory reset the watch to clear the update, but this repeated a few days later. I would come home with the watch at 25% for a couple of days and it would do the same thing. Again, it was downloading it over the LTE connection.
I finally disabled WearOS from using background data, and so far that has stopped it from using my cellular data to download. I just flip the toggle from bluetooth to WiFi when I get home. However, it's incredibly annoying that there isn't a way to say "No, I don't want to update". I know most of the time we want updates, but this is absurd. Worse, because I did a factory reset it used nearly 700MB of cellular data in the process. I didn't go over, but that is a pretty large, unexpected, hit. I'm close to rooting the thing just to put an end to it.