I’d recommend asking the admin of the site in question, they can delete anything and generally they’re willing to. They can even hide revision history on a post-by-post basis if you need to remove personal info.
When it comes to your rights as a user it starts to become a bit of a legal question, and this varies a lot by region. In some places an admin is legally obligated to fulfill a deletion request.
Our default settings are cautious in favor of the health of the community. If a very active user decides to delete all their content this could be damaging to a community and can render a lot of discussion unreadable.