Previously, bug 200039 was filed (and resolved fixed) to enable the flag, but it doesn't seem like we ever actually enabled it? Digging through the history, it appears it's always been off. That said, the only place we actually check that it's enabled is FloatBoxExtent PrintContext::computedPageMargin(FloatBoxExtent printMargin), so it's practically enabled anyway.
<rdar://problem/118773100>
Pull request: https://github.com/WebKit/WebKit/pull/20872