Plugin to clear topic's custom field when timestamp gets changed

Is it possible for plugin to act on topic timestamp change? Is there an example of it?

Take a look at DiscourseEvent. You can tie into those via plugin and do stuff when they happen. You can find the list of event by searching the repo.

The ones that may apply to what you’re looking for:

  • post_destroyed
  • topic_destroyed
  • before_create_post
  • before_create_topic
  • post_edited
  • topic_status_updated