You can find the callbackId in the Conversation Details (GET /api/v2/analytics/conversations/{conversationId}/details).
In the customer's participant structure (purpose = customer), you will have a session structure of mediaType = callback (in the sessions array attribute). The sessionId attribute corresponds to the callbackId needed in the bulk disconnect.
You can also find it in the Conversation Context (GET /api/v2/conversations/{conversationId}).
In the customer's participant structure (purpose = customer), you will have a callback structure (in the callbacks array attribute). The id attribute corresponds to the callbackId needed in the bulk disconnect.
As I said, I. just run a test. Request without callbackId (didn't cancelled the callback) - Request with callbackId (did cancel the callback).
You can open a ticket with Genesys Customer Care to report the issue on the API endpoint definition.
The description is provided by the Engineering team in charge of this endpoint, and then automatically processed to be displayed on the Developer Center site.
If there is an issue with it, a ticket is necessary to have them fix it.