Had been down for a year after their ...issues.
Back up now: https://www.irs.gov/individuals/get-transcript
Now it asks to verify not only email and previous tax return data, but also "an account number from either a credit card, auto loan, mortgage, home equity loan or home equity line of credit" and "a mobile phone number with your name on the account".
Data is pulled from Equifax for the account number verification.
Holy smokes!!! TYVM
Literally thought I'd imagined the site it was gone so long! Although it didn't like any of my account numbers, which was annoying.