Hello @Scott Buratt
There is not a configurable setting for this.
The fundamental concept of a backlog is that it is work that remains to be done. There is no need to display work that is already done in the backlog. Your backlog would eventually grow to be an unmanageable list.
What problem are you trying to solve by seen closed issues in the backlog?
You could use an Advanced Search Issues filter to find all the closed issues that have no value in their Sprint field and use the Bulk Change feature to update the Sprint field in all the issues to a Sprint of your choice. If that is an "active" sprint, then you should see all the closed issues appear on the Board, I think. But you won't be able to see them in the Backlog section, only in a Sprint.