It should be a deal-breaker, in my opinion, if it didn’t have it, since many cheap Android phones do.
If the iPhone 9 has the same modem as the 8, based on the 2x2 MIMO rumor from Ming-Chi Kuo, then to me that spells trouble for that model (no gigabit in a $700 device is bad enough, especially when the S8 in early 2017 had it).
It would be inexcusable for a 2018 device not to have band 71.