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Unless Apple makes it available for your use, no. Every system has its own system font and you can find them easily on the web.

Linux doesn't have a "system font" unless you mean:

* the console font

* one of the various "fixed" fonts that come with X11

* any of the default X11 fonts

None of which are likely to be used as the default for title bars, menus, etc.

OS X and Windows have different “default” fonts too. Obviously in this case people mean the font used in the UI.

As many different "UIs" as there are that may be tricky to get a consistent answer for.

Oh well. Something else to roll into systemd!

The browser’s UI, of course.

Linux doesn't have anything it is a kernel.

Graphical distros have a standard font in their UI.