It seems that 'edit' and 'delete' both internal notes and replies to customers, have all been lumped together. Allowing agents to edit, or delete replies that have been emailed to a customer, is a terrible idea. One cannot rewrite history. On the other hand, allowing them to edit or delete their own internal notes, makes a lot of sense - even just for typos etc - but more importantly to fix inaccurate info. Permissions should be split such it's possible to only inhibit changes to things sent externally.
Split permissions out such it's possible to only inhibit edits to replies sent externally. - Community / Feature Request - Deskpro Support
I agree with this request, it may also be nice to have a limit to how long after the note creation edits are allowed. So users can't edit notes from last month or even last week.