There will not be a direct replacement for this endpoint. Unlike GET /api/task/listCompleted, which returns all completed tasks for the identity that called the endpoint, this endpoint will list all completed tasks for any identity that has at least one. If you need to get completed tasks for other users, you can use get-completed-work-items | SailPoint Developer Community with the ownerId query parameter.
GET https://{tenant}.api.identitynow.com/v3/work-items/completed?ownerId={identityId}