Yes, some social media platforms do notify you when someone leaves your private story. For example, on Snapchat, if a person decides to stop viewing your private story, you may receive a notification indicating this change. However, not all platforms have this feature. It depends on the specific design and privacy settings of the platform you are using.
One way could be if you don't see their name or profile associated with your story anymore. Maybe they were regularly viewing or sharing, and now that stops.
You could tell if someone leaves your private story when you don't get any feedback or response from them that you typically did before. Maybe they don't mention it or show any interest when you bring it up. Sometimes, it's just a gut feeling based on the change in their behavior towards you related to the story.
It depends on the platform. Some platforms offer this functionality. They might show you a list of who is currently viewing your private story and if someone leaves, that change will be reflected in the list. However, not all platforms are the same. Some may not provide any indication at all that someone has left your private story.