Posts in discussion forums that have been deleted are being included in email summaries even if the email summary is generated after the post was deleted. Only the first n characters are displayed in the email, but this is still a potential issue if the post that was deleted was inflammatory or contained sensitive data, etc.
I do not have an answer for you but if this is happening it would need to be fixed by Canvas Support. Have you reported this to either your school's Canvas team or to Canvas Support?
