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:

kintoneHelp: Can data related to deleted users still be used?

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