It is a bit hyperbolic, but both the EF, E and Nikon G mounts have been essentially fully reverse engineered as far as driving a lens. The issue is that the communication to the body of some information isn't fully understood yet and might never be, which prevents third-party lenses as well as adapters from working properly.

If you were to make a camera that is fully open, maybe even modifiable on the info it sends to the lens and what information it wants back, then you would get aftermarket adapters for Canon EF (you could also make one yourself with publicly available info). Sony E would be a tougher nut to crack, but there is a fair bit of info already out and there are already E mount adapters feasible.

So yes, maybe trivial is hyperbolic because of the Sony E mount, but very feasible.

I agree the mounts have been mostly reverse engineered, but that doesn’t detract from my point (which I might not have done a good job of articulating). The point I was trying to make is that each mount has different focal characteristics which would make it non-trivial to support on a single body (with a swappable collar adapter). Doable, yes, but not trivial. Even ignoring the focal differences, electrically (aka lens to camera communication) wouldn’t be what I would call trivial either.

(For background, a friend and I worked on designs for making an open camera platform a few years ago)

Oh, I didn't mean that the physical mounts have been mostly reverse engineered. I meant that the camera→lens communications have been fully to almost fully reverse engineered.

Mount optical characteristics are very simple. Just make sure your base mount has a larger diameter and a shorter flange distance, and you're golden. If you are designing your camera around that it is almost trivially done.

The code for implementing a basic E mount is on github, for the EF mount you just have to go looking around on some forums or buy it from the guy behind Metabones, same for Nikon G.