The IRS website still has instructions on how to pay with cash:

In person at one of our IRS Taxpayer Assistance Centers (TACs)
Call 844-545-5640 to schedule an appointment at an IRS TAC that accepts cash.
You should call 30 to 60 days before the day you want to pay.
IRS TACs that accept cash have security checkpoints to ensure your safety. Our specially trained staff will help make the process as efficient as possible for you.

https://www.irs.gov/payments/pay-your-taxes-with-cash