Be careful. Shreddit does not get rid of all reddit comments, only the ones on your profile. Reddit stores a list of the last 1000 comments and posts you've made to your profile and that list is what Shreddit and 99% of reddit 'deletion' tools use. To truly remove all your comments and posts from reddit, look into reddit-shreddit [0]. Despite the similar name, it is completely different and leverages all your posts and comments from a reddit data request to delete all of your reddit posts and comments. It is infact the only method I have found, except for perhaps a GDPR request, that is truly a complete deletion.

However, this only covers the deletion from reddit itself. There have been periodic reddit archivals ever since it was created. Up to 2015 they were done yearly, ever since they have been done monthly. Since they have always been up for download, all of your reddit data will still be out there, however you can remove your data from the main source of these archives called pushshift. I haven't done it personally but I've seen it said on reddit, you can email the dev of pushshift or dm him on reddit and he will remove your information from his archive.

[0] https://github.com/nixfu/reddit-shreddit