I have labels for each topic and I also have a label called “read” for papers I have already gone over.
Is there any way I can view the topic label but exclude the read label so I only see unread papers from that label?
At the moment you would have to create a new label to reflect the exclusion. To do so select all papers with label A and assign them a new label B. Then select all papers with label A and the “Read” label (you can do this by clicking the label and the shift-clicking the second label). With only these labels selected, remove label B from them.
Note that this would not be automatically updates, so it is most useful for one-time searches.