Disclaimer: I am not affiliated with Let's Encrypt, so I don't know this for sure.
Based on what I've seen in recent Boulder developments such as this issue, it's the latter: Let's Encrypt will remove the email addresses from their own databases, but sign them up at an external mail provider. This allows them to send general, untargeted emails to subscriber with announcements, without actually linking an ACME account and an email address.
As per the issue linked above, the current intended design appears to be to forward them to the external mail service, and then immediatly "forget" them. Thus they will still appear to be supported.
Interesting; I thought they were going to remain to alert users about revocation. Did you see anything about potential callback urls taking their place?
The ACME ecosystem is getting increasingly complex now - some providers will require a contact, and now some will (essentially) do the opposite.
This will have interesting impacts upon unsubscription behavior as currently one can simply update the contact to nothing/blank to effectively "unsubscribe". I wonder what will happen with such behavior after this point?
Let's Encrypt will not retain emails in the CA. We'll send a welcome email with information about staying in contact with Let's Encrypt, including opting into mailing lists, for new users who subscribe with an email.
Any idea what the operation will be when changing/removing email addresses in accounts? I suppose if the addresses aren't kept connected to an account then there won't be a way to change/remove them later. Seems like a one-way street. Suppose that unsubscription and resubscription can just be handled at the email list level via links in the email (or via the link that @mcpherrinm provided above). Just doing a little pre-thinking on how I'm going to word this in CertSage. I think the current wording probably works fine based on my assumptions here. I'd need to remove the unsubscription line and the button wording will need to change.