Will my subscribers know something went wrong?

This is about what your subscribers see if your own Benecaster licence lapses — after a failed payment, say — and the grace period runs out. See Payment Failure and Recovery for that timeline.

They will see their private feed stop working, but nothing tells them why. Benecaster sends them nothing, and their podcast app can only report that the feed failed — it has no way to distinguish a billing problem on your side from a show that has simply stopped publishing. If you want them to understand what happened, you have to contact them directly.