This is pretty common in my experience, but often with 
    slightly different terminology. [...]

    I've also seen chat functionality where users had 
    the ability to delete messages but admin users had 
    the ability to view

Great points! Slack actually works that way, if I'm not mistaken. Admins can read deleted messages and, I think, the edited versions of messages right? Not sure how they store things internally, but you're right - it's common functionality in many classes of apps.

    In my experience soft delete is basically free if you do it from the start.

This is my experience too.