T-Mobile CDMA network shutting down soon

They also inherited a small CDMA network from MetroPCS, which they wasted no time sunsetting once they turned Metro into their budget brand.

But they don't have to implement CDMA2000 like other countries' models even though it uses Qualcomm chip.

That's a hard question to give a definitive answer to. In many ways, the answer is "no". However, as with most things in tech, there are always edge cases.

Sprint's network (which T-Mobile bought and is integrating into the T-Mobile network) never went fully VoLTE (voice over LTE) so if you had a phone that didn't support CDMA voice and you were on the Sprint network experience and you traveled to one of the rare areas where Sprint had coverage and T-Mobile didn't and there was no Sprint VoLTE available...

The thing about wireless is that there's always some vocal person who is getting better service from something that is worse for 99.9% of people. Of course, 99.9% means that you have one person out of every thousand and if you're a wireless carrier with over 100M customers, that means 100,000 people might be annoyed by progress.

In America where there's a huge rural population that expects all the amenities of big cities while the nearest neighbor is a quarter mile away, that can create some issues.

Plus, it's not just the 3 national carriers. There are also smaller carriers like US Cellular whose VoLTE support is newer, Shentel (who T-Mobile has more recently bought and whose network will get integrated probably in 2022/2023), etc. Those smaller carriers may not have upgraded all of their network yet. If you're European, the idea that we chopped up our licensing into hundreds of tiny license areas might sound weird (734 cellular market areas for the initial licenses, different sizes/shapes for subsequent licensing). VoLTE interoperability has been a bit of a nightmare and so CDMA has offered a certain amount of fallback for roaming (though that situation has mostly gotten cleared up, partly because T-Mobile didn't really have legacy compatibility to fall back on).

Finally, is it worth supporting? Ultimately, the support comes for free given that Qualcomm's X60 modem was going to support it whether Apple wanted that support or not. I think the iPhone 14 will support CDMA as well simply because Qualcomm's X65 modem will support it. I'm guessing that CDMA support will be dropped with the iPhone 15 when Apple moves to their own, in-house modems.

Verizon is shutting down their CDMA network at the end of 2022. By the time the iPhone 15 launches in fall 2023, CDMA will be quite dead. But legacy things stick around for a long time because even when it's not used by 99.9% of people, there's always some that will vocally complain about change and regulators don't like the idea of customer equipment being rendered useless.

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.