This question is often asked when interviews fall in April or May, right after a new tax filing season deadline. The guidance for interviewing officers is found in the Foreign Affairs Manual. It indicates that the most recent tax return is the one filed prior to signing the I-864. It does not indicate a newer return must be supplied at the interview if a new tax season has closed before they got around to interviewing you.
Here is the whole document for reference:
9 FAM 601.14 AFFIDAVIT OF SUPPORT https://fam.state.gov/search/viewer?format=html&query=I-864&links=864&url=/FAM/09FAM/09FAM060114.html#M601_14_13
Selected excerpts include:
9 FAM 601.14-6 REQUIRED SUPPORTING EVIDENCE
b. Required Documentation: The sponsor(s) must provide the following documentation to satisfactorily complete Form I-864:
(1) Sponsor's Federal income tax returns for the most recent tax year:
(a) Each sponsor must submit with Form I-864 a photocopy or Internal Revenue Service (IRS)-generated transcript of the most recent income tax return that the sponsor had filed prior to the time of the AOS signing.
9 FAM 601.14-16 CHECKLIST FOR PREPARING THE AFFIDAVIT OF SUPPORT
d. Documents for the Principal Immigrant and Accompanying Dependents:
(1) Principal Applicant:
(a) Original or copy of Form I-864 and Form I-864A, Contract Between Sponsor and Household Member (if needed); must be signed (not required to be notarized).
(b) The sponsor's most recent Federal income tax return filed prior to the time of Form I-864 signing is needed for each principal immigrant.