I guess they are still working on ACME v2 API support in Boulder, which is why new API endpoint is not available for testing yet: https://github.com/letsencrypt/boulder/projects/1.
Note that ACME v1 API won’t be disabled immediately - as we read in ACME v2 API Endpoint Coming January 2018:
We will be adding a new ACME v2 API endpoint alongside our existing ACME v1 protocol API endpoint. We are not setting an end-of-life date for our ACME v1 API at this time.
so existing clients would continue to function.
Also, developers of ACME clients can already use Pebble, which is a “small ACME-08 test server”. As Let’s Encrypt strives to be standards-based, there is nothing which prevents developers from implementing latest ACME specification in their clients.