According to SemVer2:
4.Major version zero (0.y.z) is for initial development. Anything may change at any time. The public API should not be considered stable.
5.Version 1.0.0 defines the public API. The way in which the version number is incremented after this release is dependent on this public API and how it changes.
There are plenty of 0.x.y packages in METADATA.
They are all to be considered Alpha.
They don’t have to follow most of SemVer. In particular they are allowed to make breaking changes in patch and minor versions. Though I would personally suggest they do it in minor versions, like JuliaLang/julia does.