As of this writing, the latest releases of Darwin are 20.2.0 (stable) and 20.3.0 (beta), respectively corresponding to macOS 11.1 and macOS 11.2 beta. However, the `doc-operating-system-*` strings defined in tmac/mdoc/doc-common-u only count up to 19.2.0.
Obviously, keeping this list up-to-date is an exercise in futility, as actively-maintained platforms wheel out new releases all the time. I don't know how important these string definitions are for backwards compatibility, but .Os should at least accept an unrecognised version number without complaint. mandoc(1) already trusts the *version* parameter of the .Os macro, even if it's from an ostensibly non-existent version. Groff should do the same; even for historic platforms, validating version strings is outside the scope of a macro package's responsibilities.
