To clarify, UMA does not appear to be at risk in any way.
Every article I've found says Tmoible@home is being discontinued. That is the SIM in the router, not UMA.
Articles also say that "T-Mobile was careful to note that this will not impact its Unlimited HotSpot WiFi calling service."
T-Mobile to discontinue @Home service « Boy Genius Report
T-Mobile discontinuing @Home calling service - FierceWireless