No one would opt in, and phone GPS would suck. GPS used to take minutes to gain a lock, before they started using cell towers and WiFi points to speed up locks. Put your phone in flight mode, reboot and try get a lock, it will take forever (you may have to do other things to clear your cached location).

The link has instructions

Well, @passenger@sopuli.xyz mentioned that it’s on the google page. But then when I chose some of the brands under “Find specific steps for your access point”, it’s a bit useless because manufacturers web admins have no sense of discipline or self-control. They cannot resist the urge to rearrange their website which broke Google’s links. But further below on Google’s page are some general instructions. I would try to follow those. And if you get stuck, maybe try posting a support request in !homenetworking@selfhosted.forum.

(update) I just had a closer look at the general instructions to do a quick sanity check. Google’s instructions for linux are bad at the 2nd step. That is, running ifconfig as a linux user can be tricky.. might need to run /sbin/ifconfig instead. And even if it runs, it does not give the gateway anyway.

If you’re on linux, run ip route instead, which will say something like default via 192.168.1.1. Whatever IP address is the default gateway, that is what you need for the next step. Note I am only talking about linux. Google’s instructions are probably fine for other platforms.

You're not wrong, but in this one instance you're not entirely right, either. If your SSID is set to broadcast then you're sending it out there for anyone to see and use however they see fit. You're basically opting in by that alone. The fact that most WIFI routers are set to broadcast by default and most users have no idea how to change it is another argument entirely.

I’m glad that opting out is available, but there is no practical, technical enforcement.

What sort of technologically-enforced mechanism might you envision that does not rely on legal enforcement?

Lately I practice the only tech-enforced option I know of: wholly pull the plug on wi-fi. Indeed, this means I only have ethernet in my house and wifi radios are disabled. It also means for my smartphone to reach the cloud, I am reverse tethering over USB. Try getting a crowd of people to do that. I will praise you if you can get 10 people to do that.

There is such a thing as bluetooth routers. So a middle ground would be to pull the plug on wi-fi and use bluetooth instead. Though it’s a compromise because we cannot¹ be certain that Google does not also harvest bluetooth. But at least the limited range would mean fewer cases where the signal reaches the street. Of course you would have to be okay with the slower speed.

If Google violates their own policy, it’s legally actionable. So if you are going to run wi-fi you can do your part in helping grow the legal liability that Google has signed up for. I see no good excuse for not appending _nomap to the end of your SSID. It’s just pure lazyness not to.

¹ well strictly speaking, we might know from the location data whether Google uses bluetooth. But if it’s not part of the location data it would not be an absolute indicator that it’s not collected.

Glad to hear Apple has aligned with Google on that.

It has nothing to do with your phone. If you have wi-fi at home and the wireless signal reaches the road, then it’s relevant.

Did you follow the link? It’s Google’s own policy.

I’m not seeing a good case here for not putting Google in a position of violating their own legally binding terms.

Not to use what? Wi-fi? Of course, if you have no need for an access point, not having one is the best case.