The goal is to track which assigned tasks have been completed. Although the resolution status of topics is displayed in the list, adding a filter would be better, just like GitHub’s Pull Requests page default to having X Open and X Closed. (If possible, it would also be great to be able to filter items that are incomplete but closed separately, and to change the default to only show items that have a specific value in assign_statuses .)
I noticed that in the category where solution types can be selected, there is a dropdown in the upper right corner for filtering by resolution status, and the Where topics option in Advanced filters also supports filtering by resolution status. Therefore, the inability to differentiate based on this in Groups feels like something is missing.
After some understanding, this involves the interaction of two components, so it might be somewhat difficult?
Regardless, I’m listing the requirement here first, looking forward to good news from the Discourse team. 
Regarding why unassign_on_close cannot be used to solve this:
unassign_on_closeis completely different from this requirement. It merely removes and records the assignment relationship between the assigner and the topic, but the topic will no longer appear in the assigned list of the assignee or group until the topic is reopened (ifreassign_on_openis enabled).