the WWDC announcement said it would be mandatory starting in the spring. did that change?

Mandatory so long as the app supports other third party authentication. If they exclusively use only their own accounts, it isn't required. (There are a couple other exceptions for business and education apps)

Edit: https://developer.apple.com/news/?id=09122019b

New Apps will be required to implement this immediately, existing apps have until April next year to support it before updates will be rejected.