This is such a bummer. Given how many firmware update changes Apple has made in recent OS releases, it's a drag that they have not added any support for newer cards to the EFI.
And I read elsewhere on this Mac Pro forum that Apple no longer supports mixing cards from different manufacturers in Mojave? So I can't take out my Sapphire RADEON HD 7950 Mac Edition and put my GT 120 back in and hook it up to a small 7" monitor (for boot screen support) and slap in a nice, recent model AMD card alongside? And, conversely, I can't put the GT 120 back in and slap in a nice recent model nVIDIA card (because no Web drivers in Mojave)? Thanks a lot Apple, thanks a lot ...