When a Microsoft 365 account that collaborated a file with you is deleted, the file will go into read-only mode, and then eventually will be deleted. Using the following approaches will help ensure that you retain access to files you need.

When possible, collaborate on files in Microsoft Teams. Collaboration in Teams ensures files are not tied to a single account, so the departure of a Team member won't require ownership of files to be reassigned. For more information, see Why Use Teams File Storage? and other articles about using Teams with documents and files. 

Regularly look at your list of files shared with you in OneDrive, to make sure none of them are owned by someone who has left. Files shared with you can be reviewed in OneDrive on the web.

Remind people who are preparing to leave Cornell to transfer files that may be needed after their departure.