In MDL-48838 we moved grade_categories cache to MODE_REQUEST to prevent issues using MODE_SESSION as we could invalidate the user performing the grade_categories data update, but not another user's cache containing the same data that is invalidated. Eloy wisely commented about it in https://tracker.moodle.org/browse/MDL-48838?focusedCommentId=400500&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-400500.

There is a preformance benefit in using session instead of request.