Which releases are supported with semantic versioning of modules?
You got an email because the 2.0.0 version has been marked as unsupported by the project maintainer when, on the Releases edit form, the Supported checkbox for that version has been deselected.



On Drupal.org, 2.x, 2.0.x, and 2.1.x are considered like three different branches, and have each the setting to make them supported or recommended.
It's not just a matter of version number, but also what the project maintainers mark as supported. Probably, if you had version 2.1.0 installed, that were marked as unsupported, and version 2.0.0 were marked supported, you would get an email telling you the version you installed got unsupported.