is there a way to disable update notifications in iWork apps (Pages, Keynote, Numbers)?
Yes, you can disable this. It is very easy, although completely unsupported and undocumented by Apple.


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


These instructions are summarized from a wonderful post on MacRumors.