Hi @mzimmerman , thanks for your reply.
If I removed them from the course, would that delete their forum responses and assignment submissions from Canvas? We still need to be able to access them as administrators for archival purposes only, but just not have the Teacher of the course see them.
Note that prior to posting this question, I already created a Completed section in the course and moved all previous participants to that section. Then I set the enrollments for current Teachers to "Limit this user to only see fellow section users". The Concluded students' Discussion posts are still visible to the Teachers, even those their assignment submissions are not.