How to keep the identities although the account is deleted from Source of Truth

Make sure you set the appropriate Content-Type header. For example:

curl --location --request PATCH 'https://devrel.api.identitynow.com/v3/sources/2c9180857be05639017be4c71b414094' \
--header 'Content-Type: application/json-patch+json' \
--header 'Authorization: ' \
--data-raw '[
    {
        "op": "replace",
        "path": "/description",
        "value": "testing triggers"
    }
]'