To confirm, will we be migrating from slim 2 to slim 3? Since that is a major and important upgrade. I think slim/views does not support slim 3. We are left with twig view or PHP view. Which one should we integrate?
Rather than upgrading libraries in this project directly, T114969: Update grantreview codebase to use the wikimedia/slimapp library should be done first. wikimedia/slimapp could be updated to use newer libraries. There may be a task somewhere about the slim2 to slim3 migration for that library. The view layer used by wikimedia/slimapp and its dependent projects (Wikimania Scholarships and several Toolforge tools) is Twig.