Per the fine, fine folks in Redmond:
Upcoming, we will be retiring the preview program enabling REST API access to on-premises mailboxes for Hybrid Exchange organizations.
Note: This will impact those who write code against Exchange mailboxes.
- Timing: Beginning early March 2023, we will begin to return errors for any requests made for mailboxes that remain on-premises.
- Action: Use Microsoft Graph for Exchange Online and Exchange Web Services (EWS) for Exchange Server on-premises
- Roll-out: tenant level