My phone at the time worked fine on 4G for over a year, but suddenly one day it no longer worked once they started enforcing this. I suspect the carrier wanted to collect a troll toll from phone manufacturers to allow them the privilege to sell a phone to their customers

Its certainly possible that they’re trying to extract a toll from handset manufacturers, but I could also see it being a spectrum consolidation. Can I ask if your OnePlus 5T was a model specifically made for the USA market or was it imported from China or Indian markets? I’ve seen non-domestic model phones not contain all the same radios as North American phones. So while its possible there were a few specific bands overlapping that allowed it to work, those bands could have been deprovisioned from phone service or sold off to other companies wanting to buy spectrum.