According to a recent tweet by a T-Mobile rep in response to a customer asking when the new Lumia phones will arrive on T-Mobile’s network, the carrier replied by saying that the new phones will be exclusive to AT&T. According to the rep, it seems that Microsoft decided to scale back retail availability, so AT&T will be the only carrier you will be able to purchase them from.
It is unclear if this is a timed exclusive or if AT&T will hold the exclusive rights for perpetuity, but for now if you want either handset, you will have to get them via AT&T. However T-Mobile did state that customers are more than welcome to bring them over to their network. Given that both AT&T and T-Mobile are GSM-based networks, this should not be an issue.
Both the Lumia 950 and Lumia 950 XL have been pegged for a release next month, but no specific dates have been mentioned. Both devices are also expected to be the first handsets to run the Windows 10 operating system.