Q:

Can a flashed Mac Pro 2009 (5,1) support macOS Mojave

Can macOS Mojave be installed on a Mac Pro 2009 that has been flashed/upgraded to the 5,1 firmware (Mine already is) with a Metal supported GPU like a AMD RX 580?

A:

Yes, got Mojave on my 2009 5,1 flashed with the High Sierra update this morning using the 6/18 Mojave download.
The Apple store wouldn't let me download Mojave directly to my 2009, had to download on another machine (a 10.13 Fusion VM) then create a bootable USB, boot my 2009 from the USB and install from there.

