How to deactive the visual editor in notifications?

To do that, you'll need to disable it in WordPress. Go to your profile and put a check mark in the Disable the visual editor when writing box. Then when you go to the notifications, you'll no longer have the WYSIWYG, but you'll also not have it when writing a WordPress post or page.

I'll ask the developers today, but I don't know of a way to control it independently.

I am not a fan of visual editors either, but if you are having issues with the visual editor for the mail template, then you've probably experienced issues with the visual editor in making WordPress posts, and disabling it for both won't be a bad solution.

The reason it has the WYSIWYG on that field is that it was highly requested, and instead of developing our own solution, we hooked into what WordPress uses by default. Hence why WordPress' controls determine if it shows up or not.

Is there a reason why you can't do it for all of WordPress? Do you use the WYSIWYG editor for writing WordPress posts? Once you are done creating the templates, will you be changing them often? If not, you can turn off the WYSIWYG editor, do your stuff and then turn it back on once you are done.