Do you know any examples? I've literally never had or even heard of a smartphone refuse to boot up for lacking a sim. I use several of my old phones around the house as security cameras and all of them work fine without sims.

I think it is common (if not a requirement in some jurisdictions) for phones to be able to make emergency calls even if they don't have a SIM card, or don't have credit. In fact, apparently making an emergency call but hanging up before it is connected is a recommended step in some iPhone tech support guides:

https://www.ukeysoft.com/unlock/bypass-iphone-activation-wit...