If they had obtained the data without you supplying it freely then that would have been an entirely different matter, especially if it was used in ways that you did not consent to. But since that does not appear to be the case here the GDPR applies like it does to all data that is directly related to a data subject but continuing to store it on behalf of the user(s) that supplied it is not a problem.
Note that the user here is disappointed that their data which they consented to be kept is no longer there. This is a pretty clear indication that as far as they are concerned their expectation was the even with the GDPR up and running that such data would continue to be preserved as it is in almost every service that existed prior to may 2018.
It is precisely this kind of panicky thinking around the whole subject of the GDPR that gives these irrational responses, companies that suddenly no longer dare to mail you but you have to log in to their portal, which is secured by your email address and more of these totally weird constructs.
If they wanted to delete this data the better way would have been to positively contact the user (so that you know that they have received your message) to ask if their data should be deleted or not. That's good stewardship, just tossing it isn't.
I'm still kinda salty about it. I understand why big services can't retain data indefinitely, but like... it's just a few KB of text, and that text happens to have a lot of sentimental value. Besides, OkCupid knows that I deactivated my account because I am a success story -- why not hold onto those profiles a bit longer? Or better yet, how about emailing an archive of those messages immediately when you click the "I'm leaving because I'm in a happy relationship now" button? /rant