The rule was first formulated by Murray Gell-Mann in 1961[1] and independently proposed by Susumu Okubo in 1962.
However, the simplest, lowest-dimensional representation of su(3) is the fundamental representation, which is three-dimensional, and is now understood to describe the approximate flavor symmetry of the three quarks u, d, and s. Thus, the discovery of not only an su(3) symmetry, but also of this workable formula for the mass spectrum was one of the earliest indicators for the existence of quarks.
[4][5] This formula is phenomenological, describing an approximate relation between meson and baryon masses, and has been superseded as theoretical work in quantum chromodynamics advances, notably chiral perturbation theory.
The baryon decuplet formula famously allowed Gell-Mann to predict the mass of the then undiscovered Ω−.
The following book contains most (if not all) historical papers on the Eightfold Way and related topics, including the Gell-Mann–Okubo mass formula.