I posted an update on the PKP forum. It appears the API is usable again, though please read on.
Typically we would not end the life of a service so swiftly. However, we are about to release a new version of our REST API on July 13 and we can’t guarantee that this old v1 deposit API will continue to work after that date.
The v1 deposit API is deprecated and unsupported - if, during the REST API roll out, the v1 deposit API breaks, we might not be able to fix it I’m afraid.
If that happens, you can continue to export your xml, and upload it to Crossref’s systems. However, we strongly suggest to upgrade your OJS instance to a supported version that makes use of our v2 deposit API.
I’m very sorry our communications around this haven’t been better.