I am glad I trashed my reddit posts/comments before deleting accounts with shreddit [0].
It's to the point where I search for them every few weeks and take time to edit and delete them manually, after which they seem to stay gone.
My best guess is that they can detect mass deletion and have some sort of automation that restores posts at (seemingly ) random. Either that or their platform is broken enough that editing or deleting posts isn't reliably committed to disk.
There is no conspiracy to restore deleted comments lol. You can only retrieve 1000 items with Reddit's API so when you use Shreddit to delete your posts only 1000 are deleted. Everything else before that remains untouched. Use PullPush to really delete everything.
My reddit profile was empty, I'm aware of the api limitations, I read the readme.md, shreddit ran on a loop for 24 hours, each time pulling 1000 posts, editing, and deleting them. My profile is still empty, and currently my google results are empty, but in another few weeks some will pop up again, but still won't show in my reddit profile.
Clear enough for you?
Later move to shreddit and created a cronjob to delete the entries, and kept shreddit running a week or so. As you suggested, you will hit Reddit's rate limit soon after start mass deleting or your account shadow-banned.
Just checked two of my deleted account, can't see any post or comment. I wish I didn't delete it, just overwrite them with random sentences from local AI