I think one could make an argument that data Reddit has falls into the category: "Personal data basically means any information about a living person, where that person either is identified or could be identified. Personal data can cover various types of information, such as name, date of birth, email address, phone number, address, physical characteristics, or location data – once it is clear to whom that information relates, or it is reasonably possible to find out." People post all kinds of information on Reddit that leads back to their identity, making it reasonably possible to find out who they are (hence why people get doxxed). https://www.dataprotection.ie/sites/default/files/uploads/2019-07/190710%20Data%20Protection%20Basics.pdf
Edit: But that said, there are exceptions where entities may retain data in furtherance of other objectives, and erasing data isn't the same as formally requesting its removal, so while this is all super shitty and shady, it might not actually be illegal. We need the data privacy experts to weigh in!