Just adding to the previous post, I think a lot of people would be surprised at how many games have support for both Direct3D and OpenGL/Vulkan already built-in, and the user can usually switch between them via a setting in the graphics options, a console command, or by manually switching between them in a preference or .ini file in the game's folder.
Almost all games using the Unreal or Frostbite engines can do this, and Blizzard's games (like WoW), are very easy to switch from Direct3D to OpenGl and vice versa. Heck, a lot of games still run on Vulkan or OpenGL by default. In fact a lot of developers aren't actually all that thrilled with Direct3D 12 or Apple's Metal API. OpenGL/Vulkan is the only truly multiplatform graphics API.
In Elite's case, Apple strong-armed Frontier into using their new Metal API rather than OpenGL (but to be fair, Apple's implementation of OpenGL hasn't been updated in years), which at the time didn't support the compute shaders that Horizons uses to render the planets for landings, so Elite's Mac support has fallen by the wayside. But if Frontier revitalizes Mac support with Metal 2, which has what Elite needs, Frontier would be trying to support four distinct builds of the game on three unique graphics APIs. Direct3D on Windows and Xbox 1, OpenGL on PS4, and Metal/OpenGL on Mac OS X.
That's pretty hard to support as it is even before trying to include support for a hundred different Linux distros.