Safari became truly niche when they retreated from Windows. And everyone I know with a Mac uses Chrome. The only place it's relevent is on iOS, and only then because of enforcement by Apple to use their rendering engine - given the freedom, Chrome would use Chromium on iOS, and Firefox would be Gecko.
We can't even debug mobile Safari pages without a Mac. So it ends up as 'probably works' because it works in Chrome.
Safari on Mobile still has a considerable market share that is not going away. But on desktop Chrome dominates. Even on mobile, Android market share (hence Chrome) is a lot larger than Apple.
Newest Safari is only available for macOS and iOS.
There used to be Windows version of Safari, but it is not updated anymore.
Safari is based on Webkit/KTHML/Konqueror source code, some of the source code is public and has been contributed back from Apple to Webkit. But full source code of Safari is not public.
We can't even debug mobile Safari pages without a Mac. So it ends up as 'probably works' because it works in Chrome.