Following the recent announcement of grading periods in Google Classroom, Google are introducing another feature that enables teachers to customise grading in Classroom. The new option to disable submissions after a due date allows teachers to stop accepting submissions for an assignment past the due date.
When creating a new assignment, teachers can decide whether or not to require a strict due date. By default, the assignments tool will still allow submissions after the due date.
This feature also allows teachers to choose to stop submissions for an assignment at any point, regardless of the strictness or presence of a due date. For example, a teacher could allow submissions for any assignment during the semester or year, but turn off submissions once the period is completed.
Please click on the link to find out more, Google Workspace Updates: Disable submissions after a due date in Google Classroom (googleblog.com)