What functionality is lost without LTE?
Yes. 

Every iPhone has enough GSM frequencies to work with any carrier that wants to provide service and/or bill you international roaming. 

Of course, in bad coverage areas, higher frequency signals are much shorter range so the traditional CDMA frequencies work better in rural areas. 

With 250 plus carriers in 150 countries, you’ll always have some pain in some places, but an iPhone that’s still in Apple support is basically a universal communicator for social, monetary and technical reasons. 

And in fact, after looking closer at the Apple page cited, we can see that the four models OP mentioned have exactly the same 41 bands.