As far as I know there's no definitive link. There is data of neurotoxicity with stimulants and animals but the dosage given to the animals was far than that considered therapeutic to humans when adjusting for things like weight. Further there's data showing that no treatment of ADHD leads to loss of neurons.
https://pdfs.semanticscholar.org/e614/2095d05f469d7389d8a9e9f8ff939b2bcee2.pdf
This also brings up some other issues. We live in a new era where older men are exposed to commercials showing a 65 year old man cut, lean, and fit with a 20 year old girlfriend claiming how great his sex is thanks to testosterone. It could be that testosterone use could lead to serious risks.
So if you read the paper I put above notice it mentions males had increased risk of PD but not women? The authors suggest perhaps it's because boys get more ADHD treatment than girls (which is a leap that can't be proven, the authors even acknowledge so). There's other data suggesting it was really the testosterone causing problems and not ADHD meds.
Effects of Oxidative Stress and Testosterone on Pro-Inflammatory Signaling in a Female Rat Dopaminergic Neuronal Cell Line. - PubMed - NCBI
We know that abusing stimulants causes damage to the dopaminergic neurons but also know that being sober from meds for 3 years greatly lowers that risk.
I don't have a better answer for you but this is IMHO something that should be better explored.
Just wanted to add, and this doesn't answer the question in a scientific sense but does in legal sense, it's not within the standard of care to start inserting the fear of Parkinson's into a patient with ADHD who takes stimulants no more than it is reasonable to start telling anyone who drives a car that they're at risk for a serious car accident.
It does, however, IMHO create a reasonable argument that patients on stimulants, even if doing well, should be told to consider perhaps Wellbutrin and/or Strattera if they're going require long-term treatment for their ADHD.