Thanks a lot for the link! I ended up downloading the database, and going through all of my email contacts to see who was affected to write to them individually.

About 7/10 frequently contacted people were in the database (...!). About half of those let me know that the passwords were not in use anymore. The other half was very, very grateful...!

It was a great time to remind them about password managers, 2fa, etc.

Rather often quite incomplete, often only has partials of dumps he claims to have added and when this was pointed out he didn't want to add the rest - didn't even want to change the description of the dumps to say they were incomplete