The meta.SE Q & A Send authors an inbox message if their question gets closed has a good deal of support for this idea, but not much in the way of counter-argument.
For balance, I'm going to add a contrary view based on the thinking in Jeff Atwood's blog post Optimizing For Pearls, Not Sand (where questions are grains of sand that may occasionally lead to the answer 'pearls' we actually want).
Generating a notification would fall on the optimizing for sand side of the ledger.
A question that gets placed on hold is almost certainly not a great question. Maybe it could be improved to be a great question, but more likely it will consume community time (in close/reopen reviews, votes , comments, edits and so on) before ultimately becoming an OK question (or not).
That time could be better spent writing answers to other (better) questions.
Without the proposed notification, a poor question is quite likely to be automatically deleted (i.e. without consuming any community time). From the site's point of view, this is probably a good outcome.
If a notification makes any sense, it would be for relatively new questions (say less than a month old) that have been placed on hold (a state which lasts a maximum of five days). But these are exactly the sort of 'current' questions that the question author should be actively checking in on.
If the author doesn't care enough to do that, why should the system and/or community make extra effort to try to bring that sand up to some kind of minimum standard?
Now of course the people here are generally Nice: we care about people getting answers to genuine questions, even if they're not as well-written or on-topic as we would like. We like to help. But there are limits, and rightly so.