I am working with Odoo v13 Enterprise.
I noticed that an Activity can be scheduled for a User but they can CANCEL it, and there is no Chatter record that it was canceled.
I noticed that an Activity can be scheduled for a User but ANOTHER User can complete it.
I don't want to support either of these things. We are using Activities to encourage good behavior (have automated the creation of Approval Activities in some cases) and canceling them without leaving a note that they were canceled does NOT encourage good behavior. (It encourages people to bypass the Approval Activity)