You are not the owner of your content, you are the owner of your personally identifying data. So if tied to you then must go…but if you have written somthing that is not personally identifying then Reddit could keep it
The GDPR states that data is classified as “personal data” an individual can be identified directly or indirectly, using online identifiers such as their name, an identification number, IP addresses, or their location data.
And if these online identifiers give information specific to the physical, physiological, genetic, mental, economic, cultural, or social identity of that natural person.
In some circumstances, even information related to a person’s job, hair color, or political opinions could be classed as personal data. Usually, this comes down to the context in which the data was collected and whether a data subject could be directly or indirectly identifiable.
https://www.gdpreu.org/the-regulation/key-concepts/personal-data/
So pretty much any significant discourse you have on a platform would be covered under GDPR as an EU citizen. They also have a list of examples on this page too.
The website also states that „properly anonymized data“ is not affected by the GDPR.
The only things from that list, that should be posted on a public internet forum, are race, gender and political views anyways. And it isn‘t really possible to identify a single user based on these data points
By submitting content to Reddit you also granted them an irrevocable license to use it (according to their ToS) and Art.17, 3a of the GDPR protects data that is not identifiable from deletion
But I guess it‘s worth a try. Maybe their DPO is a nice guy
Okay then it wouldn’t be protected by GPDR, but instead Copyright Law (since you are always the copyright holder of your own stuff) which can’t be diminished by companies terms of usage.