It's not officially supported by JRiver, meaning they don't go out of their way to run it or support it. There's been multiple issues reported over the years, which were caused by OS quirks when running MC (e.g. running in core mode). It may be working now for you, but if I were you I wouldn't expect it to always keep working. Especially when it's recommended and just easier to use Windows 10 or 11.
Honestly, unless you're actually hosting a website or something (and even then I wouldn't recommend installing and running MC on it), there's absolutely no reason to ever consider using Windows Server over Windows 10 or 11. It's an old audiophile myth that running Windows Server OSes (or doing things like turning off/tweaking services in Windows or running "debloat" Windows scripts) somehow improves sound quality, but this is not true at all (where in the case of tweaking services and especially running "debloat" scripts you're technically breaking the OS). Modern hardware of the last 10 or so years running Windows 10 or 11 is more than capable of handling on-the-fly playback of bit-perfect audio without issue.