I think Battle Bond Greninja will be allowed in OU for several reason.
- First, it has mediocre offense stats before transform.
- Secondly, even with the Atk +SpA boost, its coverage move is still weaker than Protean STAB-boost (remember that Protean means it gets 1.5x boost on both Special and Physical coverage, while you have to choose to invest in either Atk or SpA in case of BB Greninja, which makes it even weaker if goes mixed-offense).
Yes, it has an amazing move in term of Water Shuriken after transformation, but it also means even worse 4MSS. Dark Pulse and Water Shuriken are madantory, then you may need Hydro Pump for strong Water STAB before transforming (you need to net KO to transform), which gives only 1 flexible slot for coverage. The increase in Speed is nice but won't make much difference (Weavile and Topu Koko is the only 2 significant things between 122 and 132 Speed, but after transforming you'd rather hit them with priority Water Shuriken anyways).