Outreach retains all activity its users so we can track their history. When a user leaves an organization, or a new user is taking over a previous user's account within Outreach, we recommend locking the previous user's account and creating an account for the new user. See locking a user account.
Keep in mind, Outreach does not support repurposing old user accounts for new users. If an old account is repurposed for a new user, issues will arise due to actions and history from the previous user.