The issue is that regardless of Google/Bing caches, there are sites like Pushbullet that archive ALL comments and posts that were made on Reddit, with APIs that savvy people can use to check deleted/edited comments. Awareness of these archives seems to be spreading through Reddit.
There are quite a few HN mirrors out there, too
https://hacker-news-example.factor.dev/
I'm not really sure why people would do this, except perhaps some seo shenanigans?
A motivated individual could still dig things up, but many people (even most users of reddit) are unfamiliar with the archives out there.