A bit more detail, from: https://www.androidpolice.com/2021/08/05/t-mobile-3g-shutdow... (from the update towards the bottom):

T-Mobile is no longer accepting CDMA-only devices and phones without VoLTE as of June 16.

T-Mobile's UMTS 3G network is still scheduled to close down on October 1.

Sprint's legacy CDMA 3G network is set to shut down on January 1, 2022.

Sprint's LTE and 5G network will turn off on June 30, 2022.