Whenever a rule sorts people into groups and treats one group differently from another, the first constitutional question to ask is whether that sorting is fair, and fairness in this sense is governed by Article 14 of the Constitution, which guarantees equality before the law and equal protection of the laws.
Article 14 does not forbid all classification; the State is allowed to draw lines as long as the classification rests on an intelligible differentia, some clear and identifiable basis for the distinction, and that basis has a rational nexus to the object the rule is trying to achieve. A pension cut-off date is a classic example of this kind of line-drawing: retirees before the date get one treatment, retirees after get another, and the fairness of that line depends entirely on whether the date is a reasonable proxy for whatever purpose the scheme serves, such as fiscal sustainability or administrative feasibility.
None of the other constitutional doctrines fit this scenario. The doctrine of eclipse is about reviving old, rights-inconsistent laws, the doctrine of severability is about salvaging part of a statute once another part is struck down, and legislative competence is about which legislature is authorised to make a given law under the constitutional division of powers. None of these ask whether a distinction between two groups of people is reasonable, which is precisely what is being challenged here.
So the constitutional issue raised by an arbitrary pension cut-off date is Article 14 and the principle of reasonable classification.