As in normal email communication also with support tickets it is sometimes required to send copies to third parties which should not be seen by the primary recipients. This is currently not possible. We often work around it by sending two mails: One to the primary recipient(s) and an other to the BCC recipient(s). However this bloats tickets and database and makes tickets less clear and readable.
As a request this has been denied in https://github.com/zammad/zammad/pull/1040. However this pull request is over two years old and the answer by @thorsteneckel then was, that “[…] This does not mean that there will be never a BCC functionality in Zammad. It’s only for now. […]”. Maybe/hopefully this assessment has changed in the meantime. I cannot imagine, that adding a BCC option is complicated. On the other hand I am no coder .