I see you asked this 30 minutes ago; as of this moment, their website appears to be working just fine:
https://www.irs.gov/payments
Anyhow, to answer your question, unfortunately no accommodations are made for outages according to this IRS statement (which I lifted from https://www.irs.gov/payments/direct-pay)
Please note that Direct Pay availability has no bearing on your due date, so plan ahead to ensure timely payment.
(translation: if Direct Pay is down, tough luck)
