The hardest thing of all is to find a black cat in a dark room, especially if there is no cat.
Duome.eu isn't interested in collecting any personal data and avoids it by all means; in fact, it doesn't see anything that wasn't and/or isn't still there in our public extended profiles*. At some point in the beginning we did have to keep in mind that our extended profiles used to include some social media links, but this is not the case now and there's actually nothing really personal left in the numbers or skill names or anything else that you can find in someone's extended profile at the moment.
If you're still unsure, you may wish to revise the data you provide to everyone else through your extended profile, this includes the people you follow or your bio and location details if you filled them in; you may take another step further and choose to deactivate your duolingo.com account someday — if you do so, your .eu page will be removed as well.
Lastly, if you append DUOME-PRIVATE to your full name in your duolingo.com profile, your .eu page will be blocked for everyone including yourself; if you want to bring it back, include DUOME-PUBLIC. You don't have to keep these lines forever, doume will remember your choice to help you keep your bio and location fields relevant.
Exception: if you don't qualify to be listed and (hence) you aren't listed, but you don't want to share your details, you will have to keep the DUOME-PRIVATE line in your profile forever because duome doesn't have a record for you yet and there's nowhere your preference could be stored. It has to be in your duolingo.com profile.
This feature is linked to your unique duolingo user id and your choice should be honored even if you choose to change your username at any time in the future.
I do keep in touch with Duolingo Team regarding the General Data Protection Regulation (GDPR). Should they need duome.eu to follow some guidelines, we will work to make it compliant. There are no pending requests at the moment.
Hope this helps.
* the extended profile is the public data that duolingo.com serves to everyone via the links shaped like this: duolingo.com/users/put-your-username-here