At the invoice level, you can filter out those invoices that have been written off.
This can be done by applying the following filter - "write off amount" is greater than 0 - like in the screenshot below.
This will fetch you all the written-off invoices.
Once the invoice is written off, an adjustment credit note will be automatically generated for the invoice due amount.
This credit note will have the reason code as "Write Off".
Hence to filter out such credit notes, you can navigate to the Credit notes and then filter them by status containing adjusted and Reason code containing "Write Off".
You can also export such credit notes from the Imports and Exports section under Chargebee Settings like below.