we sent an email to support@heroku.com as suggested in the docs when unable to login: https://www.heroku.com/policy/support/#:~:text=If%20you%20ar... But nobody has come back to us yet

Salesforce Service Cloud is affected by the issue too, so if they are dogfooding their own customer service software there's a good chance their support team are locked out of accessing support cases too :/

https://status.salesforce.com/generalmessages/10001540

nope, can't log in (SSO) to access the support area, sent an email to support@heroku.com