If you delete a user account, some of the data associated with that account will be deleted or become unavailable.
Note that it is not possible to undo the deletion of a user account, or to restore the deleted account or its data.

For details on data that will be deleted or no longer be available as a result of account deletion, refer to the help for each service:

Can data related to deleted users still be used?

Handling of deleted user data

If the data can be imported or exported, you can get a backup before deleting user accounts.
For details, refer to the following page.
File list for importing/exporting