Mailbird us built upon the Microsoft .NET Framework so we depend upon Microsoft to support the version of Windows as well. This means that we will at least support the versions of Windows that Microsoft itself still has extended support for. Each version of Windows have a product lifecycle after which Microsoft will no longer support it. That lifecycle can be found here: https://support.microsoft.com/en-us/help/13853/windows-lifecycle-fact-sheet
Once Microsoft no longer supports a specific version we may decide to no longer support it as well, but as we want Mailbird to be available to as many versions of Windows as possible, we only drop support if necessary e.g. to stay up to date on important security updates in later versions of the .NET Framework.
For specifics on why the base (non Service Pack 1) version of Windows 7 and Windows 8.0 is no longer supported in the .NET Framework please see this: https://www.quora.com/Why-Windows-8-is-the-only-operating-system-which-is-not-supported-by-NET-Framework-4-7-so-software-based-on-it-too-Its-both-from-Microsoft