When a Symphony user is deactivated, for example if somebody has left the company, their conversations will be removed and there is no way to retrieve them from an end-user perspective. This applies to direct 1:1 messages only.
Note: Messages from the deactivated user will remain in any chat rooms where they were a member.
If however there is a need to view a deactivated user's chat history, your internal IT team may be able to find these messages by using Content export. The chat history can then be viewed and shared as necessary.
Alternatively, an internal Symphony admin could temporarily reactivate the user's account to see if the chat history can be displayed that way and then deactivate the account again.