Hey dU,
Even if the general were dead, isn't it common practice to assimilate them into other armies? Like some parts of Ou Ki's went into Tou's and Duke Hyou's to Hi Shin's. His secrets must be known to atleast some people...
Hey fellow thermodynamic enthusiast,
I remembered that the general was the only one shown to be disfigured, so I presumed he did his research quite up close and personal. So whoever wanted to follow after him must be willing to expose themselves to poison and even after all that the poison strategy strategy got beat too.
Hell, maybe even the users in the dead general's army got exposed to it and didn't live long either.
Plus, poison use was looked down upon in the battlefield (in this series). The general only got away with it because he was some sort of high birth (if I remember correctly).
In any case, assimilating the remains of his old units I imagine would be different than assimilating a more "standard" army unit because on top of everything above, they would also need to be supplied with poison or the ingredients for it (assuming the knowedge did get passed on somehow), plus safety practices so they don't accidentally kill their own allies.
Anyway, I was just trying to provide a reasoning for the fact. It may be the case or it is also equally possible the author forget or just wasn't interested or even feeling like putting that back in. I mean, it could be as simple as: drawing a disfigured character was harder or the general's strategy wasn't very popular with the readers.
Only the author knows indeed.