Hi @RHA,
You're correct that if you delete something, students would no longer be able to access that in the Canvas UI. The thing that complicated your question a bit is notifications, especially when it comes to announcements. As an admin for a higher-ed institution, I'd estimate that over 90% of students have notifications turned on for announcements, meaning that the notification would have sent part of not the entire announcement to the students' email or mobile app. Therefore, while it's true students wouldn't see it in Canvas after you delete it, many will likely still have a copy of the announcement and be able to read it.
Hope this info helps!
-Chris