https://arstechnica.com/tech-policy/2017/12/att-says-it-neve...
As the article points out the kind of abuses people are talking about are nit theoretical; they were done before the 2015 rules went into effect. This was especially true on mobile plans.
Another thing is you can look at how this has played out in other countries, and it isn't pretty. There's an image floating around of an ad for a Portuguese ISP, which looks like a cable tv add from the 90's -- explicit lists of sites that are provided.
It's hard to study this kind of phenomenon with the level of rigor we'd expect from a physicist. That sort of thing is kinda the best you can do.
Finally, I have friends who work at companies who do larger scale traffic analysis on the net, and from what they've told me, at least before the 2015 rules, throttling was basically the norm, though you'd be hard pressed to get the pr departments to admit it.