Please be informed that when you enable the subscription features on forums, these notifications are required by the Forum subscription, this is why it is grayed out on the backend.

If you want to disable the notifications, users can do it on the front end in their notification settings. 

You can also disable the email notification but you must disable the subscription options on the forums. To disable them, you can follow the steps below:

    1. Navigate to the WordPress dashboard - Settings - Forums. 
    2. Under the Forum Features section, unmark the Subscription option and allow members to subscribe to discussions and standalone forums. You may refer to the image below.


 3. Don't forget to click on "Save Settings" below the page to apply the changes.