Whenever I am using issue tracking on GitHub with my team, should we unassign people from the issue once we close it?
I don't know which way should be done, and if there is any advantage to unassigning members.
In general you don't want to unassign an issue if it's closed.
There are two reasons why you don't want to do it:
Bonus reason: never try to optimize your process so much that it involves extra steps. Unless you have a clear reason why unassigning gives you any benefit why bother (and people will forget if you make them do meaningless administration).