Available from version 1.5.7 (r13012, codereview)
called when clearing the "You have new messages!" message, return false to not delete it
Define function:
public static function onUserClearNewTalkNotification( $user ) { ... }
Attach hook: In extension.json:
	"Hooks": {
		"UserClearNewTalkNotification": "MediaWiki\\Extension\\MyExtension\\Hooks::onUserClearNewTalkNotification"
Called from: File(s): User.php
Interface: UserClearNewTalkNotificationHook.php

For more information about attaching hooks, see Manual:Hooks .
For examples of extensions using this hook, see Category:UserClearNewTalkNotification extensions.

Details edit

$user: User (object) that'll clear the message