We just discovered that when a teacher wishes to view group discussion posts in a previous semester (closed) course, that the Group Select option is not available on the discussion page.  

The workaround is to go to People, click on the Groups tab and then open the group and click "visit".  That takes one to the group workspace.  Click discussions, then on the discussion. This is a lot of extra work, especially for teachers who create different groups for each discussion to keep things lively.

Has anyone else had this experience? 

I will report a bug to Canvas Support, but it seems there is often a problem with the Group Select visibility.  If a user has the permissions to view all groups, and discussions, they should see that option.  This is the second case we found where it doesn't appear when it used to. (The other was for TAs, who could always see it in classic discussions, but now cannot.  Apparently now we'd have to give a new seemingly unrelated permission for them to see the Group Select option, one we don't want to give.)