Thank you all for your patience. This change is related to a (now patched) ACME bug that could have potentially been used for a denial of service attack. Let's Encrypt considered it a security bug that could affect other ACME CA implementations and embargoed the information during a responsible disclosure period.
Our update regarding this ACME implementation change is available here: Change in Revocation Methods Due to a (now patched) ACME bug
Additional questions and discussion can be held here: Question re: Change in Revocations Methods