When user hits Save on Special:Preferences page MediaWiki core triggers 3 events
But all three events are triggered after setting new options in user object: https://github.com/wikimedia/mediawiki/blob/master/includes/Preferences.php#L1505
There is no possibility to compare new options with the one stored previously.
Context: we would like to track users who changed one option from enabled to disabled and trigger a bespoke action.
UpdateActually write [the PreferencesFormPreSave documentation](https://www.mediawiki.org/wiki/Manual:Hooks/PreferencesFormPreSave).