The question is not whether "spooky deanonymization as a distance" is possible; the question is how much you should proactively do to avoid this. As an extreme

The question is not whether "spooky deanonymization as a distance" is possible; the question is how much you should proactively do to avoid this. As an extreme example, if user A takes a screenshot if their own user setting page on a image sharing site, then sends it to user B that uploads it to the same site. Who is responsible for this piece of data? does share-a-pic.web need to scan all images for the username of a user that is deleting an account? This is a convoluted example where the answer is obviously no, but it shows that a purely principled answer might be unsuitable. Especially since non PII data you own mixed with non PII data someone else owns can become PII and you do not know what data others have. (you could also replace the user ID in all locations with a new tagged UUID to preserve referential integrity in the DB, but there not much more than 2^32 humans in total, it s very hard not to be deanonymizable)