In reference to:
App. 1080 - ■The Horadric monastery in Tristram is abandoned.
Where did the Horadric society go?
Are there books or reference in prior games that extrapolate on this further?
True it never accounted their reasons for departing Diablo's prison. Who knows. Maybe Diablo's taint surfaced after decades of imprisonment and Tyrael bid not them from nearing the corrupted monastery grounds. But its a potential novel (trilogy) for Blizzard to pursue latter.
No doubt the Horadrim did continue to challenge the denizens of Hell, followers of the exiled demon lords who terrorized the world since. But the Horadrim's purpose was a created party by Tyrael to subdue the threat of the banished Prime Evils. With Diablo contained their ultimate purpose, hunting down the Prime Evils was over. They dwindled down and eventually they just simply died out. Leaving Decard Cain, the last living descendant* (not a member) of the late brotherhood.
It was never stated they were to watch over them.
* Recently this may not be the case anymore.
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