They can keep it but they are not allowed to use it as of article 5&6.

Article 6 states that there are only 6 reasons why you can process data the one applying here is a

If the data subject has given consent to the processing of his or her personal data;

Informed consent has to be given freely and for each purpose individually each consent option one has to consent to has to be opt in and has to be singular not bundeled.reddit doesnt do that therfore their only right is to use the mail as part of the service. With the deletion of this account they are not further allowed to use this mail for anything else except legal.

GDPR also requieres that you use the data for the appropiate cause that the user opted in to. So if they have the data for legal stuff this would still be a violation.