Doesn't Reddit own the data now?
That's the unfortunate truth of these platforms. They offer your a platform and pay for hosting and you give them your attention and content.
Legally, they don't own the data, but they do have a perpetual license to the data and can do basically whatever they want it with it. Not much of a difference but it's one of these things where the details might be crucial.
I haven't tested it but if as a user you were to ask for your days to be deleted they'd have to comply, at least in EU. So if there was a mass walkout a lot of comments and post would disappear.