IRS notice extends the period of Forms W-4 furnished to claim exemption from income tax withholding in 2017 until February 28, 2018 and temporarily permits employees to claim exemption from withholding for 2018 by using 2017 Form W-4.
It also suspends the requirement that employees must furnish their employers new Forms W-4 within 10 days of changes of status resulting in fewer withholding allowances.
Additionally, it provides that the optional withholding rate on supplemental wage payments is 22% for taxable years 2018 through 2025.
Lastly for 2018, withholding on annuities or similar periodic payments where no withholding certificate is in effect is based on treating the payee as a married individual claiming three withholding allowances.
Pub 15 for your reference.