is there a way to disable update notifications in iWork apps (Pages, Keynote, Numbers)?

Even with App Store Automatic Updates set to OFF, all of the iWork apps pop up a modal dialog box on startup to inform you there is an update. Is there a way to disable this behavior (in El Capitan)?

Yes, you can disable this. It is very easy, although completely unsupported and undocumented by Apple.

1. Install the PrefsEditor application. (You can also do the following steps with Xcode… but ew.)
2. Open PrefsEditor and select the com.apple.iWork.Pages.plist bundle.
3. Find the TSAAUNPersistedState dictionary and click its disclosure triangle.
4. Edit the nextAlertPresentationDate field to some date far in the future (for example, change 2018 to 2030).
5. Save with Cmd+S.
6. Repeat for com.apple.iWork.Numbers.plist and com.apple.iWork.Keynote.plist.
7. Profit! (No more obnoxious popup dialogs.)

These instructions are summarized from a wonderful post on MacRumors.