The GDPR was set in place in May 2018 and since then it’s quite a debate how it’s still okay to take pictures at conferences. My solution is a variation of “offline tags”.
In 2012 a Berlin based think tank developed a system of buttons to be automatically recognised by a software and edited (either face blurred or gave a note to not take a picture. The software worked only as a prototype demonstration - but the more interesting part for events is: you can give all attendants either buttons or stickers to mark themselves as either “want to be photographed” or “don’t want to be photographed”. If there is a badge it could also be possible to have two versions of a badge with either “no photos” or “yes to photos” or some easy to understand symbol.
As a photographer I’m happy to follow these rules and will take care that no face of someone with a “no photo” tag will be in any of the sorted pictures.