Does @at-root worth it?

plainCSS.css

.org-Component {

}

  .org-Component-subChild {
    
  }
  
  .org-Component-subChild--modifier {
    
  }

sasspost318.scss

// http://sass-lang.com/documentation/file.SASS_CHANGELOG.html#318
.org {
  #{&}-Component {
  
    #{&}-subChild {
  
      #{&}--modifier {
    
      }
    }
  }
}

sasspre33.scss

.org {
  @at-root #{&}-Component {
    
    @at-root #{&}-subChild {
      
      @at-root #{&}--modifier {
        
      }
    }
  }
}}

Your idea with pre Sass 3.3 doesn't work as you wished. I copied your code into Sassmeister, that's what I got:

.org .org-Component {
  /* first level */
}
.org .org-Component .org .org-Component-subChild {
  /* second level */
}
.org .org-Component .org .org-Component-subChild .org .org-Component .org .org-Component-subChild--modifier {
  /* third level */
}

So, if you like BEM-syntax - which I don't - then the next Sass-release will possibly be great for you.

Disadvantages of this is it makes Find All really difficult. When I am a new developer on a particular project, I rely heavily on Find All to tell me if pieces of code are still being used in the codebase or used elsewhere or just to find where they're declared within a file structure I'm not yet used to.

If I saw class="org-Component-subChild--modifier" in the markup then I would search for org-Component-subChild--modifier and find nothing. This might make me think it's not being used and I would clean up the markup referencing it, or if I am sure it is being used, it makes finding the declaration more difficult than necessary.

Although this approach looks neat and saves some typing, I feel quite strongly against it.