You can pay federal taxes in cash if you want to, yes.Possible in a way, through a tax accountant or tax preparer as a proxy. If your tax preparer has power of attorney in regards to your tax issues, and makes the payment for you. You can pay the tax preparer cash, and they can remit payment electronically or using a check.
Pay with cash at a retail partner | Internal Revenue Service
The quickest, easiest way to pay IRS is to make a tax payment online. If you prefer to pay in cash, our Service providers partner with VanillaDirect to offer a pay with cash payment option available to taxpayers through participating retail stores.