If you delete a user account, the data associated with that account will also 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?

Processing deleted user data (in Japanese)

If the data can be imported or exported, you can get the backup before deleting the accounts.
For details, refer to the following page:
File List for Importing/Exporting

When I delete a user, which data items are deleted and which are not deleted? (Only available in Japanese)

If the data can be imported or exported, you can get the backup before deleting the accounts.
For details, refer to the following page:
File List for Importing/Exporting

When I delete a user, are e-mail messages of the user also deleted? (Only available in Japanese)