Now you can Complete ITR, CTR, TRT, PTR, SMSFAR and FBT 2018 onwards even if ATO services are unavailable. Once form is Completed it will be passing ATO pre-lodge in a background, so you can continue your work without interruption, such as sending e-signature request to a client.
All other forms still need ATO to be available to pass pre-lodge before Completing it.
For more details about form workflow sequence process, please refer to this guide: Form Workflow Sequence and E-signature