Official word on Insane in the Membrane

[Daewen]

Fond memories of an instance which will be swallowed up by a big dragon made of lava.

Far as I know it's not going away at all. Just, Knot Thimblejack will finally be free, and the librarians won't give rep for books. The books might even be "gone", having all been returned. Foror's Guide to Dragonslaying had already been removed from the game around the time of Onyxia's revamp to lv80. So I bet the outdoor arena and the rare mob spawn will probably stay, and all the other stuff about DM that didn't suck. YMMV.