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