My favorite obsolete-ism would be logical positivism (a statement is only meaningful if it is testable). The result was that most all metaphysical, religious, and philosophical problems vanished, as statements formulating them were rendered meaningless.

Granted, philosophers had finally talked themselves right out of a job You gotta respect that.

What killed the theory, is that the theory itself is untestable.

Although, I have to admit, I liked an anology Wittgenstien suggested (to his own work) that we could view the self-refuting theory as disposable... like a ladder that's kicked away after it's used.

As far as modern adherents to the view, I actually happen to like it a little even though the theory is for the most part dead. ... or at least I think it's still good taken in moderation, since it requires adherents of theories to pin themselves to a particular state of of the world. In most debates, the points of view have become so abstracted, often each side if talking past one another, and has set no objective criteria of the meaning of their statements.

Obscurantism is probably my least favorite ism I've run across Not sure if it's obsolete or not...