Just announced MacOS Mojave: what level of support for Metal is required? The general consensus is that older Macs can run Mojave if they have a graphic card supporting Metal (c.f. appleinsider for example). But Metal-readiness comes in different degrees. Has Apple disclosed what is the lowest bar yet?
The article Apple Outlines Metal-Capable Cards goes into the detail needed.

From linked article --


  According to Apple, the following graphics cards are known to be
  Metal-capable and compatible with macOS Mojave on the mid-2010 and
  mid-2012 Mac Pro models:  
  
  
  MSI Gaming Radeon RX 560 128-bit 4GB GDDR5
  
  
  SAPPHIRE Radeon PULSE RX 580 8GB GDDR5
  SAPPHIRE Radeon HD 7950 Mac Edition
  NVIDIA Quadro K5000 for Mac
  NVIDIA GeForce GTX 680 Mac Edition