US Congress Conference Report which has not been voted on or passed yet, refers to the final version of the bill - does not allow a deduction for prepayment of 2018 state income taxes. https://rules.house.gov/conference-report/hr-1
In addition prior revenue rulings 71-190 and 82-208 also preclude taking a 2017 deduction for payments made in 2017 to be applied to a tax liability in 2018 — notably, a year that has not even arrived at the time of payment, let alone produced any income, deductions, credits, or other items to support a tax calculation. Any payment might be in the form of an estimated tax — to be credited against the 2018 tax liability when ...