Will I have a degraded roaming experience with the AT&T iPhone 7?
CDMA is primarily used in the US. Nearly everywhere else uses GSM. If you are an AT&amp;T customer, you will not roam onto a CDMA network. AT&amp;T sells GSM phones, so their roaming partners would naturally by-and-large need to be GSM carriers as well.

Wikipedia has a list of carriers around the world with active CDMA networks and it's extremely short. Even Verizon (US) will be shutting down its CDMA network by the end of 2019.