Executed separation agreement in 2018 w/agreement to make maintenance payments upon separating. Established separate households in 2020. Are my payments deductible?

As an accountant I would say Yes, since the agreement was made prior to 2019; HOWEVER, I am not an attorney so I would need to suggest you ask for clarification from the lawyer or judge involved in your case.  

According to the IRS:

“Amounts paid to a spouse or a former spouse under a divorce or separation instrument (including a divorce decree, a separate maintenance decree, or a written separation agreement) may be alimony or separate maintenance payments for federal tax purposes. Certain alimony or separate maintenance payments are deductible by the payer spouse, and the recipient spouse must include it in income (taxable alimony or separate maintenance).”

“You can't deduct alimony or separate maintenance payments made under a divorce or separation agreement (1) executed after 2018, or (2) executed before 2019 but later modified if the modification expressly states the repeal of the deduction for alimony payments applies to the modification. Alimony and separate maintenance payments you receive under such an agreement are not included in your gross income.”