This information is not stored in headers, but there is a copy of it within the sender's mailbox. Read here for details: https://learn.microsoft.com/en-us/microsoft-365/compliance/preserve-bcc-and-expanded-distribution-group-recipients-for-ediscovery?view=o365-worldwide
BCC Field for Archived Data
I work for an eDiscovery vendor that processes data archived from Exchange for our clients. We recently noticed that these archiving tools (e.g. Digital Safe, Connected Archive, etc.) no longer populate the BCC field for emails which contain BCC recipients. Instead, the BCC info gets populated to X-MS-Exchange-Organization-BCC.
We initially thought this was an issue with the archiving tools themselves, however we have since been informed that this issue is due to a change made by Microsoft and affects ALL archiving / journaling software. Can you please provide details as to when this change was made so that we can identify any processed data that may be impacted?