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
From Metal (API) - Wikipedia:
Metal 2 is not a separate API from Metal and is supported by the same hardware. Metal 2 enables more efficient profiling and debugging in Xcode, accelerated machine learning, lower CPU workload, support for virtual reality on macOS, and specificities of the Apple A11 GPU, in particular.