can we keep 2-3 but keep 1? I understand making brutilaizer not stack with YG or BC but BC-YG isnt that bad to stack is it?

Ok, this is my feedback, wwhich will likely never bee seen or cared about but anyway...
The problem with black cleaver is that its not an AD caster item like we all thought, turns out its much better on AD carries and Bruisers. AD caster items should make their AD stacking multiplicative, like crit for Ranged AD carries.
A caster's damage is front loaded, therefore stacking reduction for the team is not as good. Casters should be selfish with their pen. This item should be flat pen + percent pen. Not reduction.
What about an item like DFG for AD, hydra's active is exactly was caster's need, not bruisers.

It should but I doubt they want to touch the 1337 pricetag.

I think it would be much more manageable, but it still comes off as the penultimate AD item.

Health for survival
Flat penetration for your squishy killing pleasure
Scaling armor penetration for your team's tank killing pleasure
50 AD to use with your armor penetrated opponent
Cooldown reduction for the offensive AND survival cherry on top of the cake.

Black Cleaver is a valid answer to almost every situation and it's no big surprise that them stacking became problematic very fast. Who doesn't want to be some armor ignoring chogath-like hp beast who also was carrying multiple BF swords?

That being said, there needs to be an item that helps ADs answer to ultra tanky built opponents in dominion if the BC is removed completely. AP Mages have plenty of items to help them get extra flat penetration along with their void staff if their opponents choose to keep stacking resists and AD mages need an option for that too.

IMO: Bring the new version in, let the flat penetration items still stack (with each other, not themselves). If someone dedicates 3 items slots to some less then ideal picks just to make armor a generally less then worthwhile option, I'm okay with that. I can build HP to outlast them if that happens. Problem with BC is that they get way too ahead in the damage / survivability arms race just by building more black cleavers.

I don't play on the PBE so I can't say with first hand knowledge but looking at the numbers I don't think this nerf will stop BC stacking. The item just has too many good stats for bruisers to pass up. Health, check. AD, check. Stacking Armor debuff to mow down tanks/bruisers, check. The CDR is just icing on the cake. Limiting the flat Armor pen is a small nerf in the grand scheme of things. The counter argument is, "Well other items do those things better. BT gives more AD, FM gives more HP etc..." But none of them are rolled into a nice neat package like BC.

Edit: After looking at math doing an extreme case of 4 BCs v BT, LW, BC, and FM the BC stacking come out with 50 more HP, and MAX CDR for the cost of 20 less AD and lower ArPen % (and no slow / LS % from FM & BT). But grabbing 2 BCs seems to be more common, from what I've seen anyway, and it's hard to find a comparable alt. build with just 2 of those other items (LW + BT is +40 AD, assuming full stacks for both examples, and better ArPen % but no health, CDR, or flat ArPen). Using a FM gives you more HP and stacked with a BT gives more AD but no ArPen, flat or %, and no CDR.

You still need at least 12k gold + boots to buy 4 BC's. It probably isn't as widespread as people are leading us to believe.

I've seen AD carries grab a BC or 2 quite a few times: Kog, Urgot, and Corki usually.