The example that sparked the discussion was Cold Comfort Farm (who reads Mary Webb today?). Northanger Abbey is another (same question re. Mrs Radcliffe). I'd say that Gulliver might count, on the basis that the travel books it satirises are now largely unread. Someone else volunteered that Diary of a Nobody began as a parody of a particular self-important and longwinded memoir, long forgotten. I wondered too about Chaucer's "Tale of Sir Thopaz" - a parody of mediaeval romances, we're told - but I think it's an open question how many people read the Chaucer without being made to.
Over to you. I feel there must be quite a few still out there!