Existing client implementations will have to retool for the V2 API regardless. There have been too many breaking changes above and beyond the new-authz endpoint.
Order based issuance in which required authorizations are returned to the client instead of the client asking ahead of time enables policy flexibility on the server side. We use this property for wildcard issuance.
The V1 API with the new-authz endpoint will continue to exist for some time. There's no planned deprecation at this point.