Because it's apparently not clear, I will try to explain again what "support" means (and yes the term is vague). For Oracle customers, a "supported" software/hardware combination means that they can expect it to work, and if it doesn't, it's a bug which Oracle has to fix reasonably quickly. Obviously "unsupported" isn't the same as "doesn't work", but it does mean that customers cannot open bugs against an unsupported product/combination when something really doesn't work.
The current list of supported host and guest operating systems for VirtualBox is listed
here. This is what's available for customers. Notice that Windows 10 is not listed.
With the freely available version of VirtualBox, the term "supported" is used in a much looser fashion because users aren't entitled to bug fixes anyway. In this context "supported" means "as far as we know it works". If it doesn't, we'll probably try to fix it, but if and when is entirely up to us.
Windows 10 is currently in a kind of twilight (or is it dawn?) zone where it's not officially supported but will be in the future. It's certainly not something we'd recommend for production systems. We obviously try to fix problems, but there can be no expectation of what and when will be fixed.
(Windows XP is in the other kind a twilight zone where it's no longer officially supported by Oracle but things that used to work will likely stay working for a while.)
) checking, together with a planned way to easily go back in case of trouble.