I don’t think Fiverr will implement this option.
Why?
Exhibit A:
Buyer asks for a logo.
You make them a logo
3 days later, Buyer edits message saying he wanted a banner.
You tell them they wanted a logo initially.
Buyer plays games with you, reports you to Fiverr Support.
Editing messages is an easy way for manipulation to happen. It would be our worst nightmare.