A decentralized system is much more like email than people may be used to. I can ask someone to delete an email and they might even say they did, but it doesn’t mean it’s true.

If I delete content on Reddit it disappears from Reddit, because there’s only one and they control it. That doesn’t necessarily mean they deleted it either, but at least it doesn’t show up on the page anymore. That also ignores that Reddit has been scraped and archived for years and years by third parties, so there is a good possibility it still exists there too. (This scraping could eventually watch lemmy too).

A public forum is a public venue, best to assume the lack of privacy.