Learn English – the opposite of “enroll”

Meta- comes from Ancient Greek, and in this sense means beyond, with, about or after. An example is Aristotle's Metaphysics which his editors placed after his Physics, and started readers thinking it might mean more than location. That suggests to me that the Latin pre- or Greek pro- could be possible antonyms used in a similar way.

But this would only help if people used it. Shall we try to push premetadata or prometadata? Or would actual data be more easily understood?

In an election, the opposite of abstaining is voting, because "abstaining" in that context means "to refrain from casting one's vote". The opposite of "not voting" is "voting".