Being an admin of an instance, I can't even see my own history of visited posts. I can't verify this, but I doubt this information is being stored in the database currently.
This being said, each instance has full control over their API server and the web-based application being served, so they could add monitoring to either to gather this data. If they did this on the API end it would be undetectable. Running your own instance is the only fool proof method, otherwise you need to trust the instance operator.