The method that I came up with to keep track of updates that students made to their Google Sites was to create a weekly update assignment in Google Classroom. Then every week students added a new page to their sites. They then had to respond to the assignment with a link to the new pages they made in their Google Sites.
In this short video I demonstrate how to keep track of updates to students' Google Sites by using Google Classroom assignments. The video shows a teacher's perspective and it shows a student's perspective of this method.