I was doing the Inverted Spire Strike today when I found a way into the Nessus open world! Load into the Inverted Spire Strike. Look slightly right of where you are looking when you spawn in, you should see a separate path from the normal Strike with some Fallen and a Servitor. Go down that path, and kill the Servitor. Look sharp right, and you should see a pair of Hobogoblins. Near or behind the Hobogoblins, you will see the white circles that propel you across large spaces. Jump into that, and you will be propelled across a large pond of Radiolarian fluid to the open world. Go and explorer more yourself!