Most deleted posts are only soft-deleted and are only visible to users with at least 10,000 reputation (who can only see them if they have the direct link) and to moderators and SE developers (who can also search for deleted posts using the undocumented, mod-only deleted:0|1 operator).
Developers, and only developers, can hard-delete posts in very specific cases, like if they contain personal information. Hard-deleted posts are nuked from the database and are visible to no one, not even 10k+ users.
In your flag history, you can see the titles of posts you flagged, and deleted posts have a red shade on them. However, you cannot see the full bodies of such posts unless you have 10k+ reputation.
However, users without sufficient reputation can still see a brief preview of deleted questions only (not answers or comments) that they flagged, helpfully or not, by hovering over the link. I'm not sure if this is by design or a bug (I'm assuming it's the former).