Right now users can show All issues, only issues in a visible epic, or only Issues without epics in a Scrum board.
There're chances that an Epic is marked as Done while some issues in it are incomplete. In this case, the incomplete issues are only visible if All issues are shown. For boards with lots of issues, there's no way to filter these issues easily.
Issues without epics won't show these issues because they're actually WITH an epic.
Create a filter in Epic column which shows Incomplete issues in Done Epics
Using Script Runner, this Quick Filter can be used: