Manual:$wgEnotifWatchlist

Details
Set to true to allow email notification for watched pages.

The option is then shown in user preferences and users can opt-in to receive one email 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 available but still disabled by default for both new and old users.

If   is false, this setting is ignored.