HTC launched the HTC 10 this spring to mostly positive reviews, a pleasant departure from several years of lackluster flagship phones. However, good reviews don't always translate into good sales. T-Mobile is known for kicking under-performing phones to the curb pretty quickly (see: the Priv), and it looks like the same thing happened to the HTC 10 last month. Almost no one noticed, though.
The HTC 10 launched at T-Mobile on May 18th, and it was dropped about the same time of the month in July. So, that's two months—only half as long as the BlackBerry Priv lasted at Tmo. That's not great news for HTC, especially considering AT&T never sold the 10.
The T-Mobile website no longer lists the HTC 10 at all in searches. If you go to the original URL, you're just redirected to a search for HTC phones. The only one available is the Desire 530. A few stores might still have some in stock, but no promises. I checked the stores near me and they were sold out of the 10 with no expectation of getting more. HTC still sells the 10 directly, but it's priced at $700. That might be part of the problem.