Summary

The Hide completed issues older than option in the general settings for Kanban Boards are limited to 1, 2, and 4 weeks. To see issues older than these time windows you're forced to use the show all option that overloads the Board.

Proposed solution

Let the user customize the time for completed issues that teams want to have visible on their Board. Another solution would be to expanding the options for this function.

Workaround

As a workaround, you can use the show all option, and adjust your Saved Filter to exclude unwanted issues by including an (e.g.) resolved >= -10w in your JQL.