Our brownout yesterday was very successful, giving us lots of useful data on which clients do and do not support async finalization. Based on that data and the great conversations in this forum yesterday, we've decided to cancel the rest of the prod brownouts, and to indefinitely postpone enabling asynchronous finalization in prod.
This change is one that we'd still love to make someday, but we have multiple other public-facing changes that we'd like to make this year. Those other changes are higher priority, and so we'd rather focus our efforts on ensuring that those changes go well.
If you are a client author, please continue work to support asynchronous finalization. It's necessary to be compliant with RFC 8555, and to interoperate with ACME servers other than Let's Encrypt.
If you are a website operator, please update or change your ACME client to one that supports asynchronous finalization for the same reason as above.
We'll leave async finalization enabled in Staging for an extended period, to allow client authors and website operators to test and verify that their clients work with it.
Please keep an eye out for further announcements regarding the other changes we intend to make this year. Exciting times are ahead!