Jump training is the same as all other training, provided the jump is kept relative to the pups age and growth and no sudden hard awkward landing then it is good exercise.
You have to be sensible about it all, never overface a pup or make it difficult, leasure jumps are fine but dont try to make a pup jump too high for the sake of it, gates and high fences are for the fully mature pup.
I have used this for nearly 30 years and never had any problems even retired lurchers still jumping for fun.
I have always been able to tell if my dogs are in pain, not showing pain because of pack weaknes