Thanks for reaching out, to your question –
You should be able to get information about merged incidents from the
GET /incidents endpoint (list of incidents over API).
Merged incidents are marked as resolved by merge, this can be found in the API responses under the
resolve_reason field with the type
merge_resolve_reason which would also provide an incident reference with the incident ID of the remaining incident.
i.e. If you are merging incident A and incident B, getting the incident details of incident B would show that it was merged into incident A.
Hope this helps clarify things, please let me know if not or if you have any further questions.