The File ▸ Export ▸ as Scrivener 1/2 Project... menu command (the Win|Mac numbering is different in the menu command, but both refer to the same format with regards to compatibility), creates a project that requires at a minimum, Scrivener 2.7 on macOS, and 1.9.5 on Windows/Linux.
That said, so long as your Mac can upgrade to at least macOS 10.14,[1] then you can run v3 on your old Mac, which will be compatible with the latest version for Windows, too—what you’d be getting with the bundle.
If your Mac truly is that old, that it cannot upgrade to within the modern security patch support window, then forget about cloud sync. You don’t want to connect an ancient computer to the internet, unless you are interested in helping to run bot nets, or enjoy fending off malware.
Technically it can be run on 10.13 as well, but none of us have anything that runs lower than 10.14 any more, so all we have is hearsay that it works. ↩︎