So, I've been googling but can't find any single instance where Apple seems to have threatened a security researcher. I'm new here on HN, but I don't understand why I was down voted. Was it seen as passive-agressive to ask, I could understand that, but any actual source would be appreciated.

We all know that anything that paints Apple in a bad light travels faster than the speed of said light and generally gets picked up by a dozen news outlets, no matter how rumour-esque it is. So it ought to be some traces of Apple taking legal actions against people reporting security issues against them, no?

In general "citation needed" on its own is frowned on as it is lazy and negative adding little to the discussion. My guess is that this is the reason for downvotes.

Some claims do have to be called out and this might have be one of them. An indication that it doesn't match your experience and that you have done at least a cursory search before requesting the citation would be less likely to be downvoted.