From Bloomberg on Dec 18, not sure if it made it into law though.

Supplemental Wage Rate Changes
The supplemental wage withholding rate, which applies to nonregular “supplemental” wages paid by an employer such as bonuses and commissions also are to change under the bill.

The supplemental wage withholding rate for supplemental wages less than or equal to $1 million, using the flat withholding method, would increase to 28 percent under the bill. The rate is an increase from the current supplemental wage withholding flat rate of 25 percent, in effect since 2007.

Supplemental wage payments in excess of $1 million during the 2018 taxable year would be subject to a reduced rate of 37 percent under the bill, compared to the current rate of 39.6 percent.