Yes, Exchange/Outlook and office365 do this for Sent Items.
The RFC says "Contains addresses of recipients of the message whose addresses are not to be revealed to other recipients of the message."
It is only prohibited to show the BCC contents to other recipients.
It is not prohibited to show the BCC contents to the sender of the message, since he is not one of the "other recipients of the message" as the RFC states.
What this means for outgoing messages to the recepients: Nothing, keep the current method.
What this means to the single message that will be placed in my Sent Items folder: Add the BCC addresses as a BCC-header.
Please (re)consider it, so the other day we can still see who we have mailed to.