The system preference OverdueNoticeBCC was changed to NoticeBcc in bug 14536, but the manual still refers to the first in the implementation checklist chapter. Also the scope of the preference was changed - instead of only applying to overdue notices, it applies to all kinds of notices now. So we also need a slight rephrase! https://koha-community.org/manual/20.11/en/html/implementation_checklist.html?highlight=overduenoticebcc
Did you get this fixed Ava?