Basically the old accounts couldn't be carried over and so users wouldn't be able to log in anymore to delete their comments.
GDPR made it more expensive to port the comments, because it insisted that the user have control over their content. It did not prevent it from being done.
Might be the copyright reform, but comments should still be fine with that.
FWIW, I completely agree with you on the chilling effect of upload filters. But upload filters aren't part of GDPR by any stretch of the imagination.
It also might not.
It might be worth it to some company to determine which it is, but who would expect that company to be Microsoft?
MSDN comments still exist and Microsoft operates several forums. Perhaps GDPR is a questionable excuse for some lazy sysadmin not wanting to migrate comments, but it doesn't fly here.
The idea is to balance the need to preserve history (community interest) and the need to protect users (individual interest). We don't get there perfectly, but I can tell you for sure that the vast majority of people who ask us about this end up happy with how we help them.
It was astonishing because of what the lawyers said: yes, it is a public relations risk to have ordinary employees communicating directly with the developer community, and we are willing to take on that risk if it dramatically improves our satisfaction metrics in the developer community.
We were basically told "don't be stupid, don't share corporate secrets, comport yourselves professionally, keep it on-topic, and if there are legal problems, legal will handle them appropriately".
I don't know that things would be the same today; it was a different time. But I'd hope so.
Unfortunately "right to be forgotten" has been wrongly defined in GDPR, if someone published something in a publicly available place, this shouldn't be removable on demand. I am perfectly for forcing companies to be obliged to delete its customers data if customer does not want make business with a said company or does not want to allow company to use the data for its business (like advertising), but this should not cover every random forum in the Internet.
In fact this is the reason those independent forum will disappear soon or later and be moved to Facebook, which is rather sad and shows only how EU bureaucracy is inefficient - they wanted to restrict Facebook usage of private data, instead they only caused even more people to move to Facebook and give up their privacy.