I would expect searching for deleted:1 to return no results here as I don't have the rep to view deleted posts, however I get loads of results returned, none of them appear to be deleted, nor have 'deleted' (the word) within the content.
What I actually seeing when I search for deleted:1?
@psubsee2003 is describing what I noticed too: When a option is ignored, and it is the only option used to search, the search returns all the posts. The difference with "deleted:" and other options, is that when other options are ignored you notice that.
For example, if I use "votes:2" the result page shows this.
If I use "votes:q123" the result page doesn't show any option as being used for the search.
With "deleted:1" there isn't any way to know if the option is applied, or not. I can know it from the background shown for the posts.
When I use "deleted:1" or "votes:q123" on this site, I get 135,086 results in both the cases; when I use "user:me votes:q123" or "user:me" on the same site, I get 1,185 results in both the cases.
I am moderator on Drupal Answers, and on that site I am allowed to search for deleted answers.
votes:2q34returns all questions (effectively ignoring the filter since it doesn't make sense), whereas searching forvotes:234returns 64 questions (everything with more than 234 votes). Just a guess though based on my experiences, so I'm not confident enough to make this an answer.unicorns:yeswill return zero because it is searching for the exact string of "unicorns:yes" rather than treating "unicorns" as a filter and "yes" as the criterion.