+Notifications

• the notification option is enabled automatically for all users and users cannot opt-out.

It's the only one type of notifications in Special:Preferences that users cannot opt out of it.

User talk page messages are the most important things in the on-wiki communication with someone. Other users count on someone being reachable on their talk page even if all other communication methods fail. Therefore it makes sense to not be able to opt out of them.

• the notification doesn't differentiate on types of updates that are done on User talk page, i.e. "add a new topic", reply, deleting topics/replies, adding wikilove - all actions will have the same wording: "[a username] left a message on your talk page in [topic title]"
• when an update involves deleting of a topic or deleting of a reply, having a notification to say "[a username] left a message on your talk page" s might be confusing for users

I don’t think MediaWiki core or Echo really knows what happens, they’re only aware that something happened on the talk page, and that thing is not a minor edit from a bot (bots have the nominornewtalk user right). Maybe DiscussionTools can help out here. (There’s a precedent for DiscussionTools enhancing Echo notifications: T253082.)