On October 24th an update was published on the Microsoft 365 Roadmap indicating that “Authenticated Received Chain (ARC) is now enabled for Office 365 hosted mailboxes.”
The rather terse update describes the benefits ARC could bring to the users of Office 365 who receive email that has been handled by intermediaries. But it ends with text that may indicate that, for now, it is only being evaluated for messages sent between Microsoft systems and/or customers. “Initially ARC will only be utilized to verify authentication results within Office 365, but plan to add support for third party signers in the future.”
This link should bring up the entry on the Microsoft 365 Roadmap.