Like any part of the OS, Apple can change or modify the preference keys. Anecdotally, I can certainly remember people commenting that some keys have been removed, so that setting a particular preference no longer works.
For instance, Apple changed
PMPrintingExpandedStateForPrint2 at some point. This is the key that specified whether the print dialog is expanded or simplified.
As mentioned on this site before, Apple does not document many of the available keys; most of which have been found by inquisitive people searching for text strings in executables. So a list of changes seems unlikely.
defaults write command does not check for validity. So I can add the key
IamFabulous -bool true to Preview's plist, and it will sit there doing nothing. Preview will never check for the value of that key. Any 'defunct' key/value will act similarly.