I think 13.0 is a good occasion to fix https://jira.xwiki.org/browse/XWIKI-13794.
Most code in pages is designed to expect the action to be == ‘view’ when viewing a page, but it’s not the case when viewing a page revision (it’s “viewrev”) so it always comes as a surprise when it’s badly displayed.
ViewAction already support the rev URL parameter but currently we redirect to viewrev action, so I would like to propose that we simply deprecated /viewrev/ action and always use /view/. In practice, it means doing the opposite of the current behavior and redirect /viewrev/ to /view/.
The only thing that will change is the redirect since the execution of the revision will work exactly the same way only with “view” as context action instead of “viewrev” so I don’t think this can really cause much retro compatibility issues.