If you're using a script to do so, make sure it's handling API limits specifically for "edit" calls. I realized after I tried overwriting mine that it was quietly skipping a bunch of comments, presumably because there is (allegedly) a 1 edit call per 5 second rate limit. Since adding a 5 second delay between each re-write, it seems to be working for me.
I ran into this issue with u/j0be's Power Delete Suite, I ended up writing my own script to do the job.
This, and also it's far harder for them to revert if they decided too much of their content had been deleted by protesting users. Overwriting the content means it remains a persistent issue well into the future, rather than something they can easily ignore or undo.
I'm not fully aware and up-to-date, but I've seen a few posts suggesting that Reddit is keeping your data on their servers, a few users have said some of their stuff got undeleted.
Yes, they keep the comment history forever. The idea is that as long as content is not publicly visible nobody will find it via Google and visit Reddit to be exposed to their ads.
This happens with PowerDeleteSuite's main version, but one of the forks has a 5 sec pause between deletes that does work. It's very slow (~1000 comments edited in 2 hours), but works reliably. https://github.com/deestan/PowerDeleteSuite
This userscript will manually overwrite comments manually. You go to your comments page, scroll until there are no more comments to load, and run it. It'll go 1 by 1 - edit, new text entered, save, 3 sec pause. By default the replacement text is a link to the script, but you can just edit that to be whatever you want (and you should edit it to make it harder for Reddit to batch restore). https://greasyfork.org/en/scripts/468337-so-long-reddit-thanks-for-all-the-fish
Note 1: Method #2 doesn't use the API, so should still work on July 1. PDS does use the API, so might not.
Note 2: Both methods allow you to continue to use the browser in another tab or window (at least in Firefox). Some browsers and extensions will suspend or unload tabs if they haven't been viewed in a while, so best policy is to do this in a separate window.
Note 3: The default PowerDelete Suite should be fine for exclusively deleting comments if you run it several times. Just keep running it until nothing shows up under comments. To be sure, once your comments are all gone, wait an hour, and check again. If anything remains, use method #2.
(Overwriting account #7 right now! Using PDS with 5 sec pause has worked flawlessly.)
All of my comments on my profile are gone. All posts. Everything.
I can google my username and still find comments out there. Yes, I can go in and manually change/delete, but they're not showing up on my profile for, let's just say, ease of removal.
This is what I did. I also added some stats about the account: account age, total karma, # of comments, ect. Otherwise it's not clear how much value is being lost if you just come across the comment in the wild.
You mentioned that so I looked back on my account that I've shredded, but not deleted, and sure enough, there was more content in it. So those have now been shredded as well.