Reddit Migration

This is a known problem: https://kbin.social/m/RedditMigration/t/47320/PSA-If-you-have-more-than-1000-posts-more-than

Also, you won't be able to delete anything in private subreddits.

Ask Reddit for your data. Until then, this might still work: https://kbin.social/m/RedditMigration/t/59451/Finally-Managed-to-erase-all-1477-of-my-comments#comments

and after you get your GDPR files from Reddit, you can do this: https://kbin.social/m/RedditMigration/t/164034/Automated-ways-of-removing-reddit-content

TL;DR: you'll need the permalinks for older comments if you want to delete them using a script. Otherwise, you'll have to google them and manually delete everything.

Also, if anyone's wondering, the format for an extracted comments.csv from the GDPR files you'll get from reddit is just a flat text file. The first line is (explicitly)

id,permalink,date,ip,subreddit,gildings,link,parent,body,media

and every entry that follows is the id, permalink, etc. so it might be possible to dummy up a comments.csv file by extracting stuff from an archive (as described in the second link above) and use the shreddit program from the third link if you wanted to delete things while waiting for Reddit to fulfill your data request.