Well, actually this could be just a means of letting people know your preference without direct communication. Maybe it could fall under existing GDPR regulation, as an extended part about a public "non consent" marker.
How would you solve the problem in large scale, low effort way?
- I was at public park. There was an event. I remember there was a warning/poster/whatever - this place features XYZ and is being photographed. If you do not like, do not participate or stand here or something along the lines.
- When kids came to my workplace as part of educational program to show how people work - we gave them out papers, adults had to give approval that their child will be photographed and photo shared on social network. If any would opt out, we would just photograph without him. I think the sole purpose of that event was to photograph on some background with national flag or something and just publish it online.
Sometimes it is ridiculous, but still this thing works like this: the school or kindergarten wants class photo: please sign here that you consent. Basically this photo is not public but limited to families for all the children that attend that class. So seems kind of too much, but ok, can live with that.
Yes! Another EU regulation will solve this right quick.