In General
Canvas categorizes students who are no longer marked as active as Prior Enrollments.
If you go the People tab in your course you should be able to switch to Prior Enrollments. 

Once there you will get a list of student names. 
Click on one and you will see these options. 
If you click Student Grades, you will get a list of assignments and grades (if you click on the assignment you will get their submission). 

In Gradebook

You can also see prior enrollments in the Gradebook

Open the gradebook tab, and expand the menu and switch to Individual View....

Now you can select Show Concluded Enrollments 

You can also sort by a student and/or assignment once you select concluded enrollments 

Once you have selected your settings you can go back to the Gradebook view