Nikon and Pentax honored their long-time SLR users.  Canon essentially threw theirs to the dogs as far as old lens compatibility goes.

That happened to me when Nikon went to auto-indexing lenses as they improved the auto exposure systems on their film cameras a few decades back. The lenses used to have a fork on the aperture ring that fit onto a pin on the outside of the camera. This was replaced by a sliding lever gizmo on the inside of the lens that moved the mechanics inside the camera. I was faced with either replacing my lenses or sending them all back to Nikon for the AI upgrade.

My gripe with Nikon is that they sell "gray market" cameras to American resellers and then refuse to stand behind them if they need service. If a sleazebag salesman swears that a camera is a "U.S." version, you or I probably can't tell whether it is or not (I know I can't) but Nikon can. And if it breaks right before an assignment and you Fedex it to a U.S. Nikon factory repair center because you need it fixed right and fixed quick, expecting to pay for the repair because it is long out of warranty, they won't work on it. In my case, after keeping it for a week or two, they sent it back, still broken, via slowest ground transportation with no comment. I then had to call the service center to find out why they didn't fix it, and some helpful employee said it apparently was a gray market camera and Nikon won't touch those. So, I then had to send it to an independently-owned repair center that works on Nikon cameras to get it fixed. Unfortunately, I had thousands of dollars tied up in Nikon lenses so I couldn't just throw the camera in the trash and go buy Canon equipment, I had to grit my teeth and buy another camera body quick to have something for the photo shoot.

I never had a Nikon film camera fail in all the years I used them and I wasted the money I always spent on a backup. So, not immediately noticing all the plastic parts on the new digital cameras, I stupidly expected that tradition to continue and put off buying a spare camera back. Lesson learned...