For anyone reading this thread;
I have been using Pages 4.3 on Sierra since I asked the question, with these ignorable minor problems: opening and saving long documents take longer, saving takes longer and produces a harmless artifact on the screen (a mysterious black rectangle.) It still has its nice icons, instead of the garish symbols that Apple is now using. All features work as you'd expect. However, Pages is a 32-bit program, and Apple thinks they upgraded it, so its days are likely numbered.
This is about the fourth time that Apple replaced a professional app with an "upgrade" with fewer features, so in the future, I won't use any Apple software that keeps its data in a proprietary format. Converting complex Pages documents to another format can degrade the layout, but otherwise it's fine.
My recommendation: Find the AppleScript that loads pages files into Pages and outputs Doc files and convert all your files. From here on out, when you save a file in Pages format, export it as a doc file. Do this now, because when Pages passes away, the script will no longer work. Paddle your lifeboat to one of the following ships, all of which can use the Doc format and have professional features: NeoOffice (app store), LibreOffice, or Miĉrosoft Word.
Pages is an excellent word processor, and I'm grieved that Apple killed it. (I bought Pages in a box for $79)