There’s a new feature in Exchange Online that automatically archives the oldest items in your mailbox after it reaches 96% of its quota. This goes hand in hand with the current time-based archiving and avoids any email disruption that could be caused by hitting mailbox limits due to big attachments or large volumes of AI-generated content. Exchange Online Plan 1 and Plan 2 will continue to offer a 50 GB and 100 GB archive mailbox respectively. This feature is currently in private preview for selected customers, will be in public preview from 15 November, 2025, and generally available in January 2026.

There are some useful FAQs and details of the announcement here: https://bit.ly/43nKeFC.