is it possible to filter a label, that is done already?
I mean to filter a task with a label that is done.
Do you mean:
a) filtering tasks that have a label such as @done?
b) filtering tasks by a particular label and seeing the tasks that are completed which also have that label?
Pretty sure you mean (b) and I am pretty sure that is not possible.
If you want to see a list of completed tasks that used to have a specific label, then you can click on the Karma score in the top-right corner, go to "View All Completed" and then filter this list by label.
However, you can't search for a specific label and from there, view completed tasks.