And when you would flip that switch, anyway? Wouldn't it make more sense to control this from your router? Something like changing the wifi password or network name would work even better (assuming you control the router.)
Maybe someone should start enforcing mandatory VPNs for households? I.e. I get you want to control your LED bulbs from your phone, but that doesn't imply they should be accessible from public Internet.
Also I wonder if the whole thing isn't just a symptom of a problem that we can no longer expand connected technologies while expecting zero knowledge from end users. Maybe it's about time to require some minimum amount of computer knowledge from citizens of a technological civilization?
You don't even need a VPN, just have the device only accept connections from local IPs (and use TCP to avoid spoofed IPs).
You need strong authentication and secure (proven input recognizer[1]) code in anything that connects to any network, even a guaranteed LAN. Border security is only one layer of protection, and LANs are rarely an isolated network.
[1] https://media.ccc.de/v/28c3-4763-en-the_science_of_insecurit...
If Internet connections worked like radio broadcasts (one-way) then we'd be fine but they are inherently two-way connections. That means that for every outbound request a hole is opened up for return traffic. Even if you think, "that doesn't count" one must still account for the fact that PCs get compromised via side channels all the time and are subsequently used as pivot points for further attacks on any attached networks.
We need to stop treating intranets like they're somehow "safe" or special places apart from the Internet. They're not.
Always treat every device on any network as if it is being placed directly on the Internet because most of the time it really is. NATs and proxies aren't security tools even if they pretend to be.
That doesn't mean that you shouldn't be able to control it all from your router too. But if your home router gets hacked, or there's a devastating crypto vulnerability, or you suspect your teakettle is ratting you out to the NSA, you should still be able to boil water with it.
Finally, a mandatory physical switch would remind people (and inform them) of just how much stuff they use is now net-connected.
I don't see any way around implementing them as bog standard "RFKill" switches. A software switch is basically the only way to go because the WiFi chips are controlled in software.
The nub of the idea is a legal requirement for a physical switch that will in all cases immediately disconnect the device from the network.