Reddit's collection of your personal data is comprehensive. Under the section "What We Collect," they detail what they collect.
As of January 2020, you can "delete" your account by going to the preference page and choosing the "deactivate" tab. They do retain personally identifiable information in order to maintain certain features. For example, if your username was "John" and you deactivate, "John" does not get deleted so that another visitor can use it.
Instead, what happens is that it gets marked "deactivated" but remains on the server so that the system knows to not allow the visitor to register with that name as it already exists. It is recommended that you delete your contents before deleting your account.
One important question to ask Reddit is if they sell deactivated user's data.
"What We Collect" https://www.redditinc.com/policies/privacy-policy#text-content1
"How do I delete my account?" https://www.reddit.com/r/help/wiki/faq#wiki_how_do_i_delete_my_account.3F