You're telling me that ADC's can be AD Casters as well? Varus, Graves, Lucian, Ezreal, and Corki all have low CD abilities that they are meant to interweave into their AA's and specifically even do things to buff the AA's in between the abilities. That sounds like the definition of an AD Caster to me but just for simplicity's sake, lets look at the group I mentioned and compare their combos to the regular AA'ing, crit based AD's.
Graves = Q, AA, E, AA, AA, Ult. There are an equally number of Abilities to AA's in this combo and the E in the middle is even specifically needed for him to get another AA off due to his reload system.
Varus = Ult, Q, AA, AA, AA, E. Again, equal number of abilities to AA's Ratio. Seeing a pattern
Lucian = E, AA-AA, Q, AA-AA, W, AA-AA. He gets more AA's in specifically because his passive encourages him to use his abilities in a controlled burst combo. :/
Now let's look at AA based AD's.
Jinx = Q, AA. That's it. You can throw a Zap in there if you want but nope. The majority is just turn Q on and AA your head off. Maybe not even that.
Tristana = E, Q, AA. Yeah, that's the most basic Trist combo. Toss a bomb on their head, press Q for an AS steroid, and try to do as much damage as possible with your AA's.
Caitlyn = E, AA or W, AA. That's the majority of her combo. The rest of it is just AA'ing with Runaans to get head shots. :/
So yes, I'd say that there are AD casters in ADCs and there are quite a lot of them at that. How many AD's are there? 18? And at least half of them I'd consider an AD Caster. :/ A good chunk of the ADC champ pool, in fact, would classify best as an AD Caster than a usual AA'ing ADC.
As for whether ADC's are abusing BoRK, that's utter nonsense. Its really good right now, yeah. But I don't feel like its outlandishly broken. I'm exceptionally happy with the change, to be honest, because it actually feels more accessible to more champions. The lowered AS sucks, sure, but that just means now ADCs and other champions who build AS can now build it without worry of going over the AS cap. Which makes it an amazing item for Tristana finally. I loved building BoRK on her in season 5 before they changed IE and Crit items so drastically. And then Season 6 and 7 happened and it became necessary for her to have two crit items to make her damage worth a damn and I just couldn't anymore. :/ it was too much AS in her build that I was wasting stats with her Q active.
Now I can actually, comfortably fit it into her build and it helps her out with that shitty fucking mid game because I get to build Statik instead of Runaan's and still get 2.5 AS with Q active. I'm finally able to win games with Tristana again, which is sad because I have 600k Mastery points and have been hard stuck and frustrated with the game for weeks because I can't viably play my main; she just couldn't function against these ADCs who were flatly better than her mid game.
I don't understand where you get the idea that ADCs are abusing this item or that they're going to nerf it; I don't feel like they are. And tbh, I hope that they don't because they nerfed Youmuu's into the ground already and made it an item unviable on any ADCs that weren't going full lethality. I used to like Youmuu's as a pocket pick for Tristana because the active worked well with her Q and it gave her a shitton of stats she needed for making it through the mid game; Armor Pen, AS, AD, and CDR so she could use her Q even more often.