Mobile operators are moving toward an all-LTE future. As more devices support LTE and VoLTE there's less need for legacy 2G, 3G and non-LTE 4G service. AT&T has already completely shut down its 2G network and is using the old 2G spectrum for LTE. T-Mobile appears to be taking a different path to all-LTE world by keeping 2G but shutting down its UMTS 3G network piece by piece. (UMTS also includes HSPA and HSPA+, which are the 3G and pre-LTE 4G technologies that T-Mobile uses.)
Two years ago T-Mobile shut down UMTS on the 1700 Mhz band in 23 Markets. Now T-mobile is replacing 1900 Mhz UMTS with LTE in 31 markets, including some big ones like New York City. Here are the affected markets and when 1900 Mhz UMTS was or will be replaced with 1900 Mhz LTE:
June 23, 2017: Allentown-Bethlehem-Easton, PA-NJ, Boston, MA, Cambridge-Newton-Framingham, MA, Cincinnati, OH, Dover, DE, East Stroudsburg, PA, Elkhart-Goshen, IN, Florence-Muscle Shoals, AL, Hillsboro, TX, Lancaster, PA, Manchester-Nashua, NH, Providence-Warwick, RI-MA, Reading, PA, Rockingham County-Strafford County, NH, Sussex, DE, Tuscaloosa, AL, Waco, TX
July 11, 2017: Long Island, NY, New Jersey, New York, NY
August 15, 2017: Atlanta, GA, Bakersfield, CA, Cape Coral-Fort Myers, FL , Detroit, MI, Naples-Immokalee-Marco Island, FL, Orlando, FL, Port St. Lucie, FL, Santa Maria-Santa Barbara, CA, Seattle, WA, Tampa, FL, Ventura County, CA
I believe that all the markets listed above still have UMTS on 1700 Mhz, which most T-Mobile phones and some non-T-Mobile ones support. However, a few T-Mobile and many unlocked phones including all iPhone 4. 4s, and 5 phones and many iPhone 5s models do not support LTE or 1700 Hz UMTS. In the affected markets phones without either LTE or 1700 UMTS support will be stuck on 2G. T-Mobile is recommending that customers in the affected markets upgrade to LTE capable phones. It's offering affected postpaid customers discounted LTE phones but prepaid and T-Mobile MVNO customers are on their own.
Sources: T-Mobile, Simple Mobile, Image: T-Mobile