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.