I have sometimes wondered this too..
So I decided this time to jump down the rabbit hole (writing this post as I go down the hole).
This is what I have found.
- The first commit of OBJLoader2 had the comment "Added Geometry2 based OBJLoader." by mrdoob as seen here.
- Geometry2 seems to have been part of some experiments to combine the ease of use of Geometry with the performance of BufferGeometry. An issue mentioning Geometry2 can be viewed here.
- There are recent commits for both OBJLoader and OBJLoader2.
- All the recent commits seem to be from a single person (kaisalmen)
So, It seems to me that OBJLoader2 is meant to be a more performant loader for OBJ files. Having a look at The main commiter's github page reveals a repository specifically for the OBJLoader2 class.
There is documentation on the OBJLoader2 class in the authors github repository here