Section 11051 of H.R. 1 reads:
SEC. 11051. REPEAL OF DEDUCTION FOR ALIMONY PAYMENTS.
(a) IN GENERAL.—Part VII of subchapter B is amended by
striking section 215 (and by striking the item relating to such
section in the table of sections for such subpart).
(b) CONFORMING AMENDMENTS.— [Omitted]
(c) EFFECTIVE DATE.—The amendments made by this section
shall apply to—
(1) any divorce or separation instrument (as defined in
section 71(b)(2) of the Internal Revenue Code of 1986 as in
effect before the date of the enactment of this Act) executed
after December 31, 2018, and
(2) any divorce or separation instrument (as so defined)
executed on or before such date and modified after such date
if the modification expressly provides that the amendments
made by this section apply to such modification.
26 USC 71(b)(2) reads:
(2) Divorce or separation instrument
The term "divorce or separation instrument" means-
(A) a decree of divorce or separate maintenance or a written instrument incident to such a decree,
(B) a written separation agreement, or
(C) a decree (not described in subparagraph (A)) requiring a spouse to make payments for the support or maintenance of the other spouse.
Linking this all up, if your separation agreement which requires you to pay alimony is executed prior to December 31, 2018, the alimony appears to remain deductible, even if the divorce decree is issued after December 31st. It also appears that you can even continue to negotiate that settlement after the 31st as part of your divorce and, as long as you do not explicitly apply the amendments made by Section 11051 to your separation agreement.
If you're going through a divorce, though, I would hope you have a lawyer. They would be a great person to ask about this interpretation.