In order to obtain a 'World Soft Object Reference' you have to add a 'Load Level Instance' node and promote the input pin to a variable. You are unable to add a reference to 'World' from the Variables Panel, only 'World Settings' and 'Material Expression' World Position.
This was found in 4.19.2 (CL-4033788) and tested in 4.20 Preview 3 (CL-4135216) and Main 4.21 (CL-4151283)
Results: You can only create 'World Setting' variable and not a World Variable
Expected: World Object Variable Type is available through the Variables panel
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-60933 in the post.