For any divorce decree that becomes final after 12/31/18, the alimony payments will no longer be deductible by the payor or taxed as income to the recipient. This only applies to new divorce decrees. Any alimony arrangement that exists prior to 2019 will be grandfathered into the new provisions.
As a tax planning move, if you are going through a divorce, and you expect you will have to make future alimony payments, you should instruct your attorney to have the arrangements finalized prior to 12/31/18. If you are expecting to receive alimony payments, and you want to avoid paying income tax on the money, you should try to delay final settlement until after 12/31/18.
The new tax law does not affect child support or property settlement payments.