> Nobody announced it

They did: https://techcommunity.microsoft.com/blog/microsoftdefenderfo...

And it appears to be further documented here:

https://learn.microsoft.com/en-us/defender-office-365/anti-p...

https://learn.microsoft.com/en-us/defender-office-365/email-...

And other sites were reporting about it for at least a year and a half:

https://powerdmarc.com/dmarc-outlook-email-authentication/ (see #4)

https://www.mailreach.co/blog/outlook-dmarc-requirements