A publisher can request notification that there are no subscribers at the time of publication. I think there's also maybe an event message that can be enabled - but I could be wrong there.
A subscription creates a topic. A subscriber can choose a time period and decide that no subscriptions have been received in that time period. But you are mostly dealing with The Halting Problem here.
You can also create a single subscription that matches every topic, and then use that to build a list of published topics - at least a list of topic strings that were published while you were subscribed.
You are otherwise trying to solve the wrong problem in the wrong way, and create a tool that monitors the wrong things.
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