Are replies to class announcements visible to everyone in the class?
What if you reply to the message that was forwarded to your email?
Also, does deleting the message make it completely invisible?
Yes, replies to class announcements are visible to everyone in the class.
If you are talking about the notification that was forwarded out of the Canvas Announcement tool via Notifications, yes, the reply should go back to the annoucement and is visible to everyone in the class.
The annoucement would have to be deleted from the course to make it completely invisible.
The only thing I would add here is that you can go to the Settings page for your course and in the "More Options" section, you have the option to disable comments on announcements.
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