AKA, the original whispersync.

Yup, this was once a thing - you didn't need wifi for sync or downloading books at all.

Kindles at one point apparently came with free cellular access

https://xkcd.com/548/

They still do - it's an option on more expensive devices (Paperwhite and Oasis).

I had a kindle keyboard and it had 3g. It worked in a bunch of countries--slowly though. I remember reading blogs where people were taking the sim cards out and tethering using them.

To save money they could come with LoRA radios and sync when the opportunity arrises to a LoRA gateway, including meshing with each other to aggregate data to increase the likeliness of encountering a gateway. LoRA modules are pretty cheap.

https://www.thethingsnetwork.org/

...which would require a valid SIM. So just don't add one. If the device comes with a pre-inserted/hardwired/virtual SIM, well... several countries in the world require KYC-style registration of the SIM owner before networks are allowed to activate the SIM, so there'd still be an opt-out path for the user in such countries.

eta: My point being: Now you're in a twisty little maze full of corner cases, all different. Not the sort of thing much loved by Amazon (or any of the GRAFT).

FWIW, the original kindle used a cellular connection to do position syncing and book downloading. No user-provided SIM needed.

Not in the IoT world. The 'owner' of the Sim, the company that sells the device, would have a deal with one or more network providers to allow access, and take care of facilitating data retention and identification regulation.

Does this work with Teslas?