Does it matter? Software will be able to change the projection to anything else.
I'm curios.
Also, take that attitude to the extreme, and you can tell people to shoot everything with a fisheye, change projection to rectilinear with software, and crop. Voila, cheap replacement to the 800mm f/5.6.
It still won’t get you the coverage of the Nikon 7.5mm or it’s replacement the Nikon 8mm full frame circular fisheye.
An EF 8-15mm f/4L and adapter combo would get one there.
Or close to the ultimate fisheye so far for FF cameras, the Nikon 6.5mm with a 220º field of view, yep, it can see behind itself! It was originally developed for atmospheric observations.
There's a Nikon F to Canon RF adapter that would allow mounting the Nikon 6.5mm on RF cameras. I'm not sure how many photographers have the deep pockets to buy it, and a real usage scenario for it, e.g. VR.