If you delete a user account, data associated with that account is also deleted.
You cannot undo delete or restore the deleted accounts or its data.

For details on data that will be deleted, refer to pages for each service.

kintoneHelp: When a user is deleted, is the data related to the user also deleted?

Garoon Help: Data Processing for Deleted Users

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

Cybozu Office Help: When I delete a user, which data items are deleted and which are not deleted? (available only 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:
Cybozu Office Help:File List for Importing/Exporting

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