Having heard the US 3G shutdown news from over here, I'm surprised they did not bother to create a workaround for unsupported smartphones. It would just have been a matter of Verizon creating a simple App to handle voice calls and text messages via 4G data to replace the default dialler App, which even the oldest smartphones would be capable of (I remember using VoIP apps back in 2012). Sure it would take a little getting used to using an App to dial out or for SMS, but better than losing voice/text services entirely until the user upgrades. It's not like the App would need to be anything special either like WhatsApp to handle multimedia, video calling, etc.
With the budget network I'm with (48 Ireland), they only support 3G roaming at present, so I can imagine this will be an issue if I visit the USA after the 3G switch-off. European law will soon require mobile operators to offer like-home connectivity when roaming throughout the EU, so it's likely they will offer 4G roaming even outside the EU when that happens. While the 48 network only offers 4G in Ireland with its partner network Three, it occasionally switches to 5G: