> Would they have zero radio coverage, or sub-optimal coverage?

Sub-optimal for data if it allows attach. If they don’t have working IMS profile it may reject the attach at worst or just have no working calls/texts at best (we do not have fallback 2g/3g except for 911 on 2g with pockets of t-mobile). There is 911 IMS profile but it sometimes doesn’t work with foreign phones, its tricky: https://optimerainc.com/wp-content/uploads/2021/11/OptimERA_...

> Could be a feature for those wanting a Wi-Fi only mobile Linux device.

I’m sorry but this is cope for a phone. At that point just buy a better linux device not built on a complex phone soc, it’s not a phone then.