It will be interesting to see how Canon implements catadioptric lenses in modern optics and digital cameras.
I experimented with a number of different mirror lenses back in the film days. After testing some 6 or 8, I finally settled on and used a Tamron SP 500mm f/8 for quite a while. Also briefly had (and wish I still had) the much rarer Tamron SP 350mm f/5.6. Those were both Adaptall lenses, so could be used on virtually any SLR mount and since the adapters are still being made so can be used on most modern mounts. In fact, I have EF-M and EF mounts that I sometimes use with a Tamron 90mm macro lens... Also have Nikon F, Konica K/AR, Pentax PK, Olympus OM and a few other Adaptall mounts for use on various collection cameras.
Yes, it was a bit limiting using fixed aperture lenses on film cameras. Shooting slides, I would switch to ASA (ISO) 200 film when I planned to use the f/8 lens. The lens came with one or two ND filters that could reduce exposure, but those were rarely needed. With Auto ISO on digital, there would be a lot more flexibility.
Eventually I replaced the Tamron 500mm with a "standard" 300mm and well-matched 1.5X teleconverter, which gave me two focal lengths in one, as well as exposure and depth of field control with an aperture diaphragm that could be adjusted. (Approx. f/4.5 to f/32, if memory serves.... less one stop when the teleconverter was used.)
There were at least two mirror lenses I know of that had variable apertures. In the 1970s Konica made Hexanon 1000mm f/8 and a 2000mm f/11 lenses, both of which had a plate in the rear perforated with a series of Waterhouse stops that could be rotated into place to change exposure. That was a pretty neat idea, though I'm sure it added to the size of the lens. These lenses also had a focusing bellows at the rear, instead of internal mechanisms. And both were expensive. An exact number is unknown, but it's believed less than 100 of the 1000mm lenses were sold. At the time they cost about the price of a small car. I never heard a price for one and none of the 2000mm were ever sold. Only two prototypes of it were made (both of which reportedly ended up in Sony's hands when they bought Konica-Minolta's photography division). I saw one of the 2000mm displayed at a camera show in Denver many, many years ago... It was huge and weighed something like 30 or 35 lb.
Most mirrorless are 250mm to 600mm focal length. Much more practical than those 1000mm and 2000mm Hexanons! They are just patents at this point, but perhaps Canon has some innovative ideas and will produce an interesting lens or two. I recall trying Sigma and Tokina mirror lenses (500 or 600mm). Also a Vivitar (not sure if it was a "Perkins") and for a while I had a "Sun" or Soligor 250mm f/5.6, if memory serves. I didn't ever test Canon or Nikon, etc., because I was shooting with Konica cameras at the time. Although I only tried out third party lenses. I once did notice a Nikkor 500mm that looked an awful lot like my Tamron... just labelled differently (obviously), fitted with a different style rubber grip and without the interchangeable mount.