Taking a look at the spec sheet for the new Nexus phones, you might notice that the LTE section lists band 12. However, this is by no means a guarantee of actual support anymore. T-Mobile has been leaning on unlocked device makers to block access to band 12 unless they get certified for VoLTE, and it looks like the new Nexus phones lack that. According to T-Mobile's Twitter account, neither phone will have band 12 enabled at launch.
The situation with T-Mobile's band 12 rollout is complicated, but the gist is that there are areas where T-Mobile only has band 12. That means in order to place a call (even an emergency one) you need to have VoLTE. The network isn't smart enough to flip over to roaming for such calls apparently. As a result, T-Mobile has asked OEMs to turn off band 12 in unlocked phones or get certified for VoLTE. There's an implicit threat attached to these requests, but it's unclear what it could really do, and plenty of unlocked phones still ship with band 12 (eg. the OnePlus 2).
@bickman2k Great questions Adam! They are not going to support Band 12, but we will continue to work with Google to enable this... 1/3
— T-Mobile (@TMobile) September 29, 2015
Smaller OEMs are likely to just ditch band 12 on unlocked devices for now rather than spend the time and money on certification. But Google? I find it hard to believe Google didn't have a chance to get its ducks in a row when the hardware clearly supports band 12 LTE. It's possible whoever is running the T-Mobile Twitter account is simply misinformed, though.
If the new Nexus phones really won't ship with LTE on band 12, that's rather embarrassing for Google. Maybe it will be added alter, but it's still pretty strange. Google might want to update the Google Store listing. Oh well... at least WiFi calling will work since it's in AOSP now.
The Nexus team is hard at work on enabling band 12 support by ship date, according to the Reddit AmA they did today: