Where did "Appendices" go? I only see "Front & back matter".

It is the same feature with a more accurate name. From version 1.9.15 the card in the book's workspace, its dialog and the option in the new-book wizard say "Front & back matter", because a Preface or an Introduction is not an appendix. "Generate front & back matter" and "Regenerate front & back matter" do what "Generate appendices" and "Regenerate appendices" did. The back matter section that is actually called "Appendices" keeps its name.