If you click the "View Diff" button when viewing a review request, it will take you to the URL .../r/<review-request-id>/diff/, which shows the latest diff. However, a URL like .../r/<review-request-id>/diff/<revision-id>/, which specifies the revision number of the latest diff would be more useful. For example, if you copy and paste the URL while the latest diff is 6 and revisit it at a later time when new diff revisions have been made, it will still take you to diff revision 6.

Tested with logged in, anonymous, and non-admin users viewing published review requests with 1 and 2 diff revisions.
Tested with an un-published draft that adds a second revision of the diff. The user who owns the draft is linked to ../diff/2/ and a non-admin user is linked to ../diff/1/.
Tested with drafts without a diff, and attachment only review requests.