Remember that you now have a situation where the vast majority of younger teachers do not know how to do mental arithmetic. As such, its use is likely to be discouraged, nevermind just not taught. Students of mine regard my mental math skills as nothing short of magic, which is concerning.
While our fleet remains grounded I'm teaching air navigation to the local ATC squadron. Lovely kids, bright and well educated -- but entirely unable to cope with simple distance/speed/time calculations. They seem to have no concept at all of solving rule-of-three problems mentally and are astonished when I do.
What's even more striking is that despite a good deal of practice, they don't seem to be getting any better. They can just about cope by the end of the lesson but by the beginning of the following week it's gone. I've thought long and hard about my teaching technique, but they seem to be able to retain other taught skills perfectly well. So I'm a bit baffled.