For example when reviewing long strings (_desc strings, for example) it can be quite straightforward to see which is outdated but fixing them may require some more time. Been able to mark strings as outdated would be helpful in that scenario, even more when you want to do this work collaboratively because in that way every translator could see which strings needs fixing.
Ok, thanks. May I ask for submitting that feature request into the Moodle Tracker (project MDLSITE, component lang.moodle.org). Please provide the description and the use cases there.
I can't promise it would be a priority but having it in the tracker will make it less likely to get forgotten.