If Reddit failed to delete data after a GDPR deletion request, then yes, they would be in violation of GDPR.
Consider requesting and then deleting your data under GDPR if you are located in the EU.
"How to Delete your Reddit Account and All Data under GDPR/CCPA" https://thomashunter.name/posts/2023-06-19-how-to-delete-reddit-account-gdpr-ccpa
Even if you've deleted you posts or account, these might be soft deletions. So you should still apply under GDPR.
Here's a template letter of how to do so. You must request your data or the deletion of you data using the correct legal framework (quoting the correct legislation) and these templates make this easy. Plus they cover more types of data than just your posts and comments!
https://www.datarequests.org/sample-letters/
(I'm reposting this info in multiple places)