They're doing this shit in Germany on their DSL customers too (the "search suggestor"). It can be turned off via some obscure setting in the customer control panel, and upon a reconnect your router gets a different set of DNS servers.
T-Mobile USA and Germany's T-Mobile are completely separate entities.
Yes, I noticed this several months ago when I switched to T-Mobile. Hoping this gains traction...
I've been a T-Mobile customer for about two years, and they've been doing it for at least that long as far as I can recall. It hasn't bothered me that much since it's not very often that I mistype a URL while using my phone without wi-fi, so I've only seen that stupid search page a handful of times, but yeah… they do it.
I ended up just installing OpenVPN client and using my existing PIA VPN subscription. You can gain a lot of metadata MITMing DNS queries...