Irs clarification:

Beginning Jan. 1, 2019, alimony or separate maintenance payments are not deductible from the income of the payer spouse, or includable in the income of the receiving spouse, if made under a divorce or separation agreement executed after Dec. 31, 2018. 

This also applies to a divorce or separation agreement executed on or before Dec. 31, 2018, and modified after December 31, 2018, as long as the modification:

• changes the terms of the alimony or separate maintenance payments; and
• states that the alimony or separate maintenance payments are not deductible by the payer spouse or includable in the income of the receiving spouse.

On the other hand, generally alimony or separate maintenance payments are deductible from the income of the payer spouse and includable in the income of the receiving spouse, if made under a divorce or separation agreement executed on or before Dec. 31, 2018, even if the agreement was modified after December 31, 2018, so long as the modification is not one described in the preceding paragraph.

See source:https://www.irs.gov/forms-pubs/clarification-changes-to-deduction-for-certain-alimony-payments-effec...