This answer came from VikingOSX regarding a similar question: "You click on the thumbnail of a Section, and Pages will present the Section panel. When you click in the body text of that section, the usual Format : Text : Style panel returns."
So, apparently new to 10.0, to reveal the "Hide on first page of section" checkbox, click on a thumbnail of the page, "Section" is revealed, and the option to hide the first page of the header presents itself. Click on the body of the text and the right pane reverts to "Text."
Thanks VikingOSX.