My preferred solution would be to keep the originals in a personal workspace on the servers. Blurring would be applied before transfer to Mapillary’s workspace so that Mapillary could only access the blurred images. As GDPR does not apply to data storage and processing for strictly personal use, this should be legal, like is storing pictures in a personal cloud.
I understand that Mapillary is not addressing the issue in the forum if they don’t have a solution ready. On the other hand, they probably underestimate the importance of this issue in privacy-sensitive countries like Germany. Not only do contributors stop contributing because of the recent legal changes but many people never joined the community because of privacy concerns.