For the right to be forgotten, this only applies to personal information. E.g. information that can be associated with information, that could be used to identify you.

Since you usually have an email for signup, that would make the data fall under personal information. But reddit could just delete the email adress and your user name and show something like:

[deleted]
When does the Narwhal bacon?

And well, it is pretty difficult to find out if, when and where there is backups that still contain your information and could be given to the AI model trainers too. To find these things out, we'd need a precedence case that makes a data protection agency investigate reddit throughouly.