Google is getting out of the business of sending email using domains that it does not administer, because it stands in the way of implementing rules that reduce spoofed email, such as DMARC. This also has the side effect of encouraging customers to host their email domains with Google.
Google is grandfathering existing email adddress configurations, but they are not allowing new email addresses to be added.
Yahoo did something similar when they activated DMARC enforcement. This made it harder to send email from mailing lists claiming to come from @yahoo.com, @aol.com, etc. . AOL soon followed suit. See this PCWorld article for more on the fallout and reasoning.
Update: There was no formal Google announcement about this policy change, but this support thread has an answer from a Google employee.