I would doubt this reaches the bar for a breach notice. Article 34 states that the breach has to be "likely to result in a high risk to the rights and freedoms of natural persons" before notifying data subjects is required.

I suspect "the right to keep your mailing list memberships and email address private" may be one of those rights/freedoms.

First one to CC-breach in a data breach notification mail acknowledging the very same breach will hold the speed record, forever.