Notwithstanding subsection (f)(2)(B) of this section, it shall not be a violation of subsection (a), (b), (c), or (e) of this section solely because a plan of an institution of higher education (as defined in section
1001 of title
20) offers employees who are serving under a contract of unlimited tenure (or similar arrangement providing for unlimited tenure) supplemental benefits upon voluntary retirement that are reduced or eliminated on the basis of age, if—