Yes, a video card that supports Metal is required for Mojave, and I think Mojave does yet another MacPro (2010 or 2012) firmware update, but don’t quote me on that. Most video cards of this nature require 2 auxiliary power cables connected, which leaves no connectors left on the Mac Pro motherboard for the extra power cable on the HDX card. There is a
thread discussing this.... just be careful you don’t fry your Mac Pro motherboard circuit board power traces, others have! I am still a bit uneasy on this due to lack of concrete information on the power consumption of the cards.
Also, you likely will want a card that is Mac ready, so you can select a startup drive on power up with the Opt key... Mac flashed, and no drivers required.