If you do not wish your students to see their grades and feedback on assignments as soon as you leave them, then ensure that your course is set to a 'Manual Grade Posting Policy' from the outset of the course. The default in Canvas is Automatic and you need to override this. Do this as soon as you start preparing your course for the upcoming academic year. Follow the instructions here: Turn on Manual Post Grades
If you do a full course copy from a previous running of the course (following these instructions relevant to our setup) this tends to bring across course settings, so if the originating course was set to Manual Grade Posting then this setting will automatically come across.
If you import individual assessments such as quizzes or assignments from other courses, then beware that these items have a separate layer of grade posting policy, and it is possible that these may be set to Automatic, independently from the overall course setting. (For more information on this see this report from Dr Rob Treharne).
The struck-eye icon is used to signify assessment columns in the Grades area or within SpeedGrader for assignments that are set to be hidden from students, in that they have not been set to automatic release of grades and feedback. This symbol does not appear until the first piece of feedback or grade is left on an assignment, and related to the last point may not reveal the individual item's grade posting policy.