T-Mobile and Sprint officially became one company this year, but Sprint's former network still exists as a separate entity. T-Mobile customers gained the ability to roam on Sprint's cell towers back in September (with the correct devices, anyway), and if recent news is correct, the legacy network might only stick around for another year.
T-Mobile has allegedly started sending out notices to its business customers, which states the company will fully shut down Sprint's CDMA network "on or around January 1, 2022." Devices that support Sprint's LTE connectivity will continue to work after that date, but CDMA-only phones won't be able to connect. T-Mobile will no longer activate CDMA-only devices after January 1, 2021, so if you're feeling nostalgic and want to put your SIM in an old phone, you only have a few days to switch over.
It's worth noting that the source specifically refers to business customers, so the shutdown deadlines could end up being different for regular T-Mobile subscribers. T-Mobile previously said that it would shut down 3G access on its own network by January 1st, 2021, shutting out devices that don't support Voice over LTE.
- Ian Littman (Twitter)