Nevermind. I figured this out. There is some code that needs to be commented out in the core code of the plugin. Since it hasn’t been updated in two years, I feel safe that my edits to the plugin won’t get overwritten.
For anyone who want to show past events with the poster board shortcode do this:
Edit the file event-organiser-posterboard.php . On line 216 comment out the first array item 'event_start_after' => 'today',
so it looks like //'event_start_after' => 'today',
instead. This is making the posterboard shortcode display only events that start after today.
Here is an example of the shortcode I used to display only the events in the year 2000 posterboard style:
[event_board event_start_before='2001-01-01' event_start_after='1999-12-31']