Manual:$wgEnotifWatchlist

Details
Set to true to allow e-mail notification for watched pages. The option is then shown in user preferences and users can opt-in to receive one e-mail when someone else changes a page they are watching.

If it's set to false, the option will not be available. If you change it to true, the option will be availaible but still disabled by default for both new and old users.

If $wgEmailAuthentication is false, this setting is ignored.