I got IRS notice -- LTR 105C - FY2021. IRS couldn't verify my federal withholdings(F-WH). Other than W2 what proof I can ask my employer, that he had deposited F-WH ?

In W2, the Federal withholding is greater than the tax I owe. I am suppose to get refund. I had already shared the W2 with them in response to CP05A. Now, I got LTR 105C saying, IRS could verify my federal withholding.