Hi @NatalieH6,
I'd say that a peer review would be a much different thing than a discussion, even though there are some similar elements. I know many faculty (even at my institution) would like to see plagiarism detection added to discussions. I'm not 100% sure why this isn't something Instructure has developed, but I can imagine the extra volume and sometimes the briefness of discussion posts could actually end up adding so much extra data into plagiarism system databases that it might make them perform worse overall. There could be other issues at play that I'm not aware of too, but I know this has been a long-requested feature so I imagine there is something holding it back.
As an alternate workaround, I know some faculty to copy/paste discussion content and submit to turnitin as a one-time check themselves. If you want to go this route, I'd suggest making sure that this would be acceptable for your school/institution, as there may be policies around using student data for something like that without each student's express authorization to do so.
-Chris
