Yup, I occasionally get replies on posts over a year old that don't show up in my history because I thought I deleted them.

Edit your posts. Don't delete them.

Nothing is 100% but my comments were first replaced with excerpts from books in the public domain, twice. Then I went back a third time and replaced the comments with gibberish, then delete. Most of the comments I find still active are just the excerpts from the books. They read like plain English, so it is very difficult for an AI, or even a contractor, to pick up on the fact that it is worthless.

Nah, reddit restoring comments is a myth. You just didn't delete all your comments, even though probably you thought you did.

See, Reddit, being the duplicitous bitch that it is, doesn't really show you all your comments when you go to your profile. It's limited to your last 1000 (?) comments or so, any comment that goes beyond that horizon is gone from your view forever, but it still exists in the thread.

The way to solve that is to first do a GDPR request. After a few weeks you will receive a zip file containing a file with all your comments and a link to it. You can then use an overwrite and delete tool and point it to this information. It will likely run for several hours or even days, depending on how many comments you made, because reddit throttles edit and delete requests, but it will effectively delete everything.