I work for a U.S.-based Fortune 500 company and I've been instructed to ask candidates during interviews, "Are you authorized to work in the United States, without need of current or future sponsorship?" My employer has a substantial legal staff; if that wording was illegal, I'm sure I would never have been told to use that question and would have been warned not to use it.
The paragraph on the I-9 form says that "refusal to hire because the document presented has a future expiration may also constitute illegal discrimination." (emphasis added) The same paragraph also has a link to the Department of Justice website, on which one can find a page "Types of Discrimination", which includes:
Employers may not treat individuals differently based on citizenship or immigration status. U.S. citizens, recent permanent residents, temporary residents, asylees and refugees are protected from citizenship status discrimination. Exceptions: [...] Citizenship status discrimination which is otherwise required to comply with law, regulation, executive order, or government contract is permissible by law.
Employers may not treat individuals differently because of their place of birth, country of origin, ancestry, native language, accent, or because they are perceived as looking or sounding "foreign." All U.S. citizens, lawful permanent residents, and work authorized individuals are protected from national origin discrimination.
However, the legal basis for those requirements appears to be 8 U.S.C. § 1324b, which only outlaws discrimination
(A) because of such individual's national origin, or (B) in the case of a protected individual (as defined in paragraph (3)), because of such individual's citizenship status.
National Origin discrimination could occur in the case of a citizen as well as in the case of a noncitizen with work authorization. If a boss turns away an applicant saying "I don't hire Irish, Mexicans or Japs", that's discrimination whether the applicant's great-great-grandparents were nationalized in 1880 or the applicant got off the airplane from said country yesterday.
Protected Individual discrimination only occurs against certain classes of people:
- Lawful Permanent Residents
- Family-Sponsored Immigrants (§ 1153)
- Seasonal agricultural workers who worked in the U.S. for at least 90 man-days during the 12-month period ending on May 1, 1986 (§ 1160)
- Refugees (§ 1157)
- Asylees (§ 1158)
If you would need future sponsorship, I don't think you fall into any of those groups. Furthermore, the inquiry about immigration status was in a conversation with a recruiter, not in the context of filling out the I-9 form itself; so the rule about "future expiration" of a document is arguably not applicable.