> What lead it to being "banned in dozens of countries all over the world, including the United Kingdom and China"?

Almost everyone who banned it did so because of acute toxicity - it requires careful handling to use safely.

Unfortunately, it was commonly used to commit suicide in many countries. In other countries, it was deaths from accidental ingestion, lung damage from unsafe handling, etc.

I don't know of any country that banned it because of a purported link to Parkinson's.