Safari has a radically different approach to numbering to all the rest (even though it’s them that changed, not it). It’s bumping its “major” number once per year, whereas Chromium and Firefox are bumping theirs 10–12 times per year. Unless they have radically different deployment strategies, “major versions” is clearly useless as a consistent concept across browsers. But despite Safari being tied to the OS version, when you take everything together (hardware support for their OSes, OS upgrade frequency) it’s only a little different, nowhere near the 10× difference in importance of a “major version”.

All up, I say “two major versions” is obviously a completely unsuitable definition to use. It needs to treat Safari differently, e.g. “two major versions of Safari and twelve major versions of all other browsers” (which would catch Firefox ESR) or “thirteen months of browser support” (which would be less for Safari most of the time, but really, consider the deployment cadence and it’s probably not so different, though maybe you’d bump it to fourteen or fifteen months, I dunno).