A couple of weeks ago some users reported a change in behavior when an Exchange Online license was removed from an Azure AD user object. It was reported that with the changed behavior mail would still flow to a mailbox, after the license was removed. When a customer opened a service request with Microsoft Support, they acknowledged the change but there was no documentation available, nor was this change announced on the Office 365 Roadmap.
The first public information appeared early October when Microsoft added the following text in the License Removal section of the Delete or restore user mailboxes in Exchange Online document.
Previously, in Exchange Online, if you removed an EXO license the user was kept, but the mailbox user was transformed into a mail user and the mailbox was moved to the recycle bin. You could then recover the mailbox within the 30 days time limit.
This has now changed in Exchange Online. If you remove a user's license, the user mailbox will no longer be able to sign in and use Exchange Online or Office 365. The user mailbox will remain in Exchange Online until it is deleted, permanently removed or purged by the Office 365 admin. You can reassign a license to the user and make the mailbox active again.
But if you look in this document today the text was removed and replaced with a link to this post on the Official Exchange Team Blog: Change in behavior for delicensed Exchange Online users
In this post Microsoft explains the change in more detail, apologizes for the way they handled the change and most important, that they rolled back the change for now:
Due to extensive feedback from customers we are rolling back this feature to the original behavior in order to improve the feature before we release it again in an easier format (and with better documentation).
For more information, read the full post here: Change in behavior for delicensed Exchange Online users