Fiverr asks that you keep all communications on Fiverr
Pros are allowed to move communications wherever they want AFTER an order has been placed.
HOWEVER, Fiverr can't see what you chat about elsewhere and chatting off Fiverr is one of the first things scammers will try to do.
Additionally, if someone pulls a bait and switch ("No I never agreed to that"), unless you have proof of that conversation on Fiverr, their support won't be able to do much. If you DO communicate with a client outside, I recommend recapping conversations in your Fiverr chat, and only after an offer has been accepted. I have had this experience on other platforms.