In Short: Google's newly released Pixel 3 and Pixel 3 XL officially have support for Band 71, where T-Mobile keeps its speedy 600MHz spectrum that it won in an FCC incentive auction, and these two devices seem to be the first phones not released by T-Mobile that can make such a boast. This, of course, means that the phones can also support applications of LTE Band 71 in other scenarios, such as with international carriers, should they roll out any equipment that uses compatible spectrum. For now, however, it seems that only T-Mobile USA has any interest in Band 71 LTE on 600MHz spectrum, and is using it for a special extended-range variant of its usual LTE coverage. The Pixel 3 and Pixel 3 XL do not seem to support 4X4 MIMO on that band, however, which means that support is mainly on board as a fallback.
Background: T-Mobile snapped up a large amount of 600MHz spectrum at the most recent FCC incentive auction, and has been putting it to massive use throughout the country. Its main efforts in 600MHz usage have been in expanding and strengthening the rural parts of its network, ensuring that no customer is left behind. The Pixel 3 and Pixel 3 XL can draw on all of that work, as stated above.
Impact: Put simply, T-Mobile customers who pick up a Pixel 3 or Pixel 3 XL won't have much to worry about as far as accidentally leaving their coverage area. Even going into a thick building won't be as bad as it may have been a few years ago, since 600MHz is able to pierce through walls and buildings better than higher spectrum bands. Customers wont' lose their connections with the outside world when they drive through bear country or walk deep into their local Walmart, but they will lose a lot of speed; the lack of support for 4X4 MIMO essentially means that Pixel 3 and Pixel 3 XL customers will lose about half of the data pipelines used by other standards in the company's arsenal. Since these two devices are seemingly the first to support Band 71 without being released by T-Mobile, a unique trait for now that will likely turn into a key selling point for unlocked devices in the US in the near future.