That would basically ban normal cell phones. You can't do calls without access to the network.
Honestly, people buying these products should be smarter. They bought an expensive chalkboard that they knew depends on someone else's computers (unless they didn't even bother to look into how the products work), without any guaranteed server support period anywhere to be found.
Anyone who cares about their purchases suddenly no longer working will simply avoid this hardware. The people who do purchase this stuff don't really care. They may act outraged in hopes of getting enough people worked up to start a class action lawsuit they can benefit from, but they failed to even consider "what if the servers shut down" before spending five grand on a glorified touchscreen and hooking themselves up to a $600 per year subscription. To make things worse, they bought this stuff from Google.
Screw Google and their e-waste factories, but banning all online products seems like an unnecessary hampering for innovation to be honest.
I suggest an alternative solution: for up to x years after a product is sold by retail stores, the servers must be up or the consumer must be able to get all of their money back from the sellers. This adds risk to the businesses launching and shutting down IoT stuff willy-nilly and helps consumers. Products sold in third party stores will have to be pretty reliable for stores to take the risk stocking this crap.
In a perfect world, I'd like the law to mandate that online-only products like these must put their server source under escrow (to be opened when the service shuts down) and must provide the user with an option to pick an alternative server address. Sadly, I don't think that idea is going to survive even a single day of IT lobbying.
With Availability as a critical part of the cybersecurity CIA acronym, I think it may just be possible that the EU's Cyber Resilience Act will include something to solve this problem.