For me "officially supported" means that MKVToolNix should run on these systems without serious limitations. I will investigate and fix bugs on those systems myself.
I will also offer binaries for those systems that I've built myself.
If an operating system or a specific version is not on the list, then MKVToolNix might still run on that system or version – but I won't spend time on fixing issues on it.
Why? There are several reasons, but the most important is that supporting systems has a cost in time. My time is limited, as is everyone else's, and this is simply where I draw the line.
Another reason may be that I lack experience developing on such a system (e.g. macOS) or lack an installation of it (again macOS, but also the various BSDs or the Solaris derivatives). Time permitting I may decide to spend time on developing fixes relating to those systems, but I cannot guarantee I will, especially as I lack systems to test those fixes on.
Supported are all client versions of Windows (meaning Intel architecture) that Microsoft supports. This means Windows 7 or newer, 32-bit and 64-bit architectures. On the server side Windows Server 2012 and newer are supported.
All versions support 32-bit and 64-bit architectures except were noted.
Arch Linux (I don't offer binaries for Arch, but this is my main development platform)
CentOS/RHEL 7 (only 64-bit; needs the EPEL repository)
I don't support macOS. I don't own a Mac, haven't used one, have never programmed in that environment and can therefore not provide support for it. I do provide binaries for it, and I may look into issues that are specific to that platform, but don't count on it.
Other systems and architectures (*BSDs, other/older Linuxes, Solaris derivatives, ARM architectures etc.)
I don't officially support other systems and architectures. This includes any type of NAS system (e.g. Synology DiskStation or QNap) and any type of Raspberry Pi and similar ones.