It may not have been "mandatory" (which kind of ties in with there not being any mandatory ground school anyway, in the PPL) but it was de facto mandatory because the nav exam had questions whose options were carefully drafted to draw in common slide rule errors where you get the answer 1 or 2 degrees out by doing the extra iterative step the wrong way, etc.
If you used a different rule, e.g. the Jepp CR-5, you could get different results because AFAICT the procedure is slightly different (there is no sliding portion). I did actually learn the CR-5 wind calcs recently but instantly forgot.
The fact that not even a £50000 slaved compass + autopilot system cannot fly that accurately is something else..........
Whether the exams have been revised to allow somebody with a trig calculator using the answer which is actually exactly correct, I don't know.
I am afraid the damage to GA as a credible leisure activity for intelligent modern individuals has been done long ago. It was well ongoing when I started in 2000. Only the most obscessive people got on with it and stuck in there. A disgrace.