As a green card holder living outside the US, are you subject to US federal taxes, e.g., global taxation?

Yes, but indirectly, since in order to maintain green card they have to be on US soil for the bigger part of the year, which implies tax residency.

That having said, not everyone spends 6 months to maintain their card and without a reasonable need to do so, it risks losing the status.

Yes. I don't need to spend time in the US to keep the green card, though, because of the reentry permit.