Earlier quoted context omitted.
Not really. There have always been smartphones that supported both GSM and CDMA. The three major carriers not only use different LTE bands, but also Verizon and shortly AT&T (in February) have adopted a whitelist model where only select handsets can be activated on their networks. It's a much less open marketplace than it was five years ago.
By select handsets, I assume any big name handset, e.g. iPhone, Pixel, Galaxy, would be allowed, but a no-name handset wouldn't be allowed? Isn't this similar to the pre-iPhone days, when if you brought an overseas GSM phone you had to go through some hoops to get it activated with Cingular/T-Mobile? I remember getting handsets from China and Europe and the reseller would have to circumvent something to get it activa…
To be frank: No. You popped your SIM in and the phone worked immediately on the network. If it didn't work you manually entered APN settings in.
Some years later AT&T started maintaining a whitelist of smartphone IMEI numbers so they could charge more for data plans than a featurephone would pay. T-Mobile did not do this. AT&T went as far as maintaining whitelisting for phones that would be allowed on wifi calling or voLTE, T-Mobile phones just needed to carry the profiles.
No 'circumvention' other than carrier/subsidy unlock was ever required to use an international phone save for the very rare times international phones had bands software disabled (and this is quite rare).