Per the Freedom Mobile Compatibility Checker, my Canadian-model Galaxy S9+ (SM-G965W) should no longer work in the USA. 3G shutdown has taken effect, and phones without "LTE Roaming" - which is somehow different than LTE "calling" - are no longer functional.
So I went to a shit-ton of work to get alternative phones lined up - an S22 and an S10+ - for my upcoming business trips. (The S10+ was the last model with a headphones jack.)
However, two separate trips from July 23 to August 04, 2022, to Philadelphia, PA and Denver, CO, show that my S9+ still works just fine on T-Mobile in the USA - even though according to Freedom, it shouldn't.
I tested the Samsung Galaxy S10+ and Galaxy S22 5G as well. They both work too.
For reference, the S9+ did show LTE when calling, so it's not likely picking up on some leftover 3G kit that hasn't been decommissioned yet. It did initially have a touch of trouble - the T-Mobile signal was weaker, so I think it tried AT&T first - but really had no issues beyond that initial hiccup.
It will register on AT&T but won't call/text, and won't register on Verizon at all, both of which are to be expected. In areas where you can't call, it's probably because T-Mobile coverage is bad, not because the phone is no longer working.
I am quite happy with this since the S22 felt like a downgrade: no headphones jack, a poorer fingerprint sensor, and a smaller screen than the S9+ or S10+, yet it does nothing that I need better than the older phones. And I'm bloody not well paying $1200+ for an S22+. I will be selling the S20 and using either the S9+ or S10+ depending on how well the S10+ fingerprint works.