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

It appears that you are using the shorter sourceId that is used by v1 and v2 API calls, but v3 and beta API calls require the longer sourceId GUID. You can retrieve your sourceId GUIDs by using GET v3/sources. The sourceId GUID is found in the “id” attribute towards the bottom of the source definition.

I tested this API call with “value”: “true” or “value”: true in the JSON body, and both worked.