Good afternoon all!

I don't have the exact date, but we did stop re-using (re-issuing, recycling) boarding pass numbers a couple years ago.

This was based on Customer feedback when someone would check-in last minute and end up in front of an a-lister, or someone who purchased Early Bird.
In order to protect your status benefits, and the purchased EB product those numbers are no longer assigned to the next check-in.
As someone noted above, they get added to the end of the C group.

Please remember that some of those empty spaces are also attributable to preboards, no shows, last minute a-listers, families traveling with children, active duty military, and others who may be invited to board earlier than their assigned position.