Assuming the answer from GDPR - is user social ID personal data is correct.
Let's consider the following case: I have a social network with public user profiles and publicly exposed user IDs. I also track user activity by these user IDs and the dashboard with user activity is also public. Then a person asks me to erase their personal data and I delete the profile, but keep the ID in the user activity table/dashboard. So new coming users cannot identify that person, but if it is an old user, he/she may maintain internal excel spreadsheet to keep the link between user ID and user data. And later on, identify the person.
Is it a valid use-case? shall I remove user ID from the user activity table?