Indeed a smartphone with no sim card is ideal for what's being discussed here, because even without a cell plan it can call 911.

Keep in mind though that even without a SIM location data will still be accessible/stored by the network providers through the devices IMEI number.

Maybe a good fix for this edge-case scenario would be an optional feature to normally refrain from connecting to the networks (i.e. airplane mode) but to override that and connect straightaway when an emergency call is placed?

I believe if you’ve turned off the phone radio (airplane mode), that should not be the case.

This would be true assuming the phone has not previously been compromised. Not a safe assumption considering baseband backdoors have been found in the wild and are suspected to be fairly wide spread. https://redmine.replicant.us/projects/replicant/wiki/Samsung...

Airplane mode

This is true -- and if you're happy with Android or iOS, then that's probably the best recommendation. But I don't use iOS, and a big part of the appeal of this for me is to be able to stop using Android.