Upon dying in a newly created world in snapshot 18w21b, I clicked "Delete World" and was taken back to the main menu yet I can still hear the game world around me including the mobs that killed me and who are actively trying to swim on where I died (meaning the world has not actually closed), also in the Singleplayer worlds menu, the world that should have been deleted still exists. Attempting to rejoin said world will show the generating spawn area text and then drop you back into the death screen.