alt Hacker NewsYou could say the same thing about the right to abortion access in the US until Roe vs. Wade was overturned (or equivalently, before Roe vs. Wade).
Comparisons to physics are unwarranted.
Laws change, and they are interpreted when applied. Very few things are black-and-white to begin with, and legality of non-disparagement clauses is one of them.
Yes, we agree on how the legal system works. Should existing laws be reinterpreted or new laws passed such that severance is treated as compensation for work already performed, then the arguments you've been making will begin to hold merit.
Until then, statements building from "severance is compensation for past labor" are imagined theories about a hypothetical reality with different laws than the world we live in.