For the Events part of the plugin (which are included on the /upcoming-events calendar) past ones only stay visible if they don’t have an end date. It has been mentioned before that’d be good if there was an option to display them still (Display old Events on the 'Upcoming Events' page), but currently it’s not possible.
And just for the extra info, the setting delete expired event posts after controls how long old calendar item posts are kept for on the topic calendars (you can see an example of one of those here - Category Calendar demo topic - calendar - Discourse Demo)
The settings specific for Events are on the second tab:
