Expansions are no longer released "solely" in physical form. Everything is downloadable now. Shivering Isles was the first of many. DLC packs are rarely standalone either.
" @Killjoi: The rule is regarding downloadable content that has been released on a disc. Since there is a "DLC Add-Ons" section on game pages specifically for that. Expansion packs that were solely released in physical form are fine. As are DLC packs that are released as standalone physical discs. "
These retail releases are always big enough updates that they require their own page. Titles like Undead Nightmare or Awakening can be listed under DLC Addons section as well but deleting them for fear of redundancy is a myopic view of how the industry works these days.