Service Operations Workspace search runs on AI Search, and closed SC Tasks and Requests are usually excluded by an active=true(or state-based) condition on the search sources linked to the workspace's search profile. To change it, go to All > AI Search > Search Experience > Search Applications and open the Service Operations Workspace configuration to find its search profile. In that profile's Search Sources related list, open the sources for sc_task and sc_request and remove the active condition from their filters. Then republish the search profile, because changes don't take effect in the workspace until you do.
Do keep in mind that these are out-of-the-box records, and customizing them will make them get skipped during instance upgrades. If you want to prevent that, I suggest you create your own search sources for those tables with the active filter and link them to that profile instead.
Hope this helps you.
