Timo Hanke’s asicboost paper explains the benefits and hardware prerequisites quite well. If I understood it right, there is no need for changing the Bitcoin protocol to enable it, it just has to be designed in the ASIC and one would need a custom miner software. So in theory all S9 could already run with asicboost and nobody would know.
But the paper also shows, that if you want the full asicboost potential in terms of silicon area and energy efficiency, the resulting ASIC could only be used with asicboost.

So if Segwit would prohibit asicboost (I don’t know if this is really the case), these kind of ASICs would be garbage.

I also guess that if somebody implemented asicboost in their ASIC (most likely anybody with <0.10 J/GH ) they probably added a kind of a “legacy” mode, which would enable a classic operation mode again, in case that somebody changes the protocol. This would cost silicon area, but still saves a lot of power.
But maybe they were just too greedy to add this backup solution.