The forum is still active. It probably does not get much use because T-Mobile does not sell Nexus devices.
You can view the Nexus 6 forum posts at Google Nexus 6.
You can view all forums on one page (this is what most forum regulars do) here: https://support.t-mobile.com/threads?filterID=all~objecttype~objecttype%5Bthread%5D
From the nexus 6 page, you can click on "update available" link in the "software info" section. This links to the 6.01 firmware. You can visit this page directly at Software versions & updates: Google Nexus 6.
Manage your T-Mobile account