I deleted all the content on my main account. Most of it wasn't useful, but a couple of posts and threads ranked well on Google and I like to think that hurts the site just a little bit.
I used a rust port of shreddit to delete all my posts across all my accounts, such that when logged in, there are no posts on my profile, however, I spent about a month on and off googling my username and finding posts that didn't stay deleted but wouldn't show up on my profile.
So I'd guess that reddit somehow restored some number of posts, and it seems to occasionally continue to do so.
In terms of hurting the site, it absolutely does. Reddit is a ghost town, and I find deleted posts constantly that google thinks are relevant. It's a shame it had to be this way.
The key thing is not to delete all your posts but overwrite a couple of times (to bust their cache). And then leave the account active with no real posts. There’s a few projects that will do this.
I wrote myself a little python script to do this a while back. I’m not sure that it will still work due to their API changes.
A longer while back I wrote a little JS bookmarklet to do it. It could just do a page at a time, which was annoying, but not too bad. However, when they would change the site, it would stop working and need to be fixed.
Remember to edit the comment before you delete it. From what I read, deleting a comment just sets a flag on the comment as delete, so it’s still in the DB for them to sell. Making it garbage text will kill the value of the comment in the DB as well, and probably really screw with the AI trying to train from it.
The best that can be done, where possible, is to edit each comment down to whitespace, save that, and then delete it. But yeah, probably still not technically good enough.
I archive (in multiple places, including locally) all of the threads that I add as sources to my personal knowledge base. I'm sorry, but shared knowledge and wisdom I can refer to in the future is more important than your "protest voice" against a centralized platform.
I recommend that if you want to delete something, then delete your account only. Your username won't be visible, but the content still will be.