As dmckee said, posts voted down to -3 or lower are shown desaturated. It has nothing to do with deletion.
If you delete a question or answer using the "delete" link on the site, it's soft-deleted, which means it still exists in the database and can be seen by 10k users and moderators, but nobody else. (There are currently around 20 people in this category, including a number of SE employees who basically never visit the site.) It's also not included in the data dump, if I remember correctly. It may be accessible via SEDE after some time, I'm not sure.
If a post includes sensitive information, such as a person's real name, address, phone number, email address, etc. (if they have chosen not to reveal that information publicly on the site), or information which is subject to a separate nondisclosure agreement or confidentiality agreement, it can in principle be removed permanently. To do so, edit the post to remove the sensitive information, then email the team (firstname.lastname@example.org) with a link to the revision that contains the information and ask them to remove it permanently. Or you can flag it for moderator attention and we'll make the edit and request for you.
It's worth noting that, while you can request to have information permanently removed, as far as I know Stack Exchange is not obligated to grant the request, because the CC license under which all content is submitted is irrevocable. (Check the terms of service, linked in the site footer, to be sure.) So be careful what you submit!