No, they just closed my bug, and I felt unable to release on catalyst :(
Recently I see they have updated the documentation for this call
https://developer.apple.com/documentation/messageui/mfmailcomposeviewcontroller/1616887-setbccrecipients with a note to say it doesn't work under Catalyst.
Annoyingly it ALSO doesn't work when running the iOS app on Apple Silicon. Which I see as a much more serious problem.
I believe being unable to put contacts into Bcc compromises email address privacy, and they really should fix this, rather than just documenting it. Please report this as a bug to them- as the more developers that do this, the more likely it will get fixed.