afni.nimh.nih.gov

"Just curious, why allow an environment variable for this? The
default (not changing my environment variables) is off, so the
new functionality is ... off."

So that the old functionality, which is to put the dataset filename in the window titlebar, is still the default. You have to request the new. On the other hand, you can do this interactively in the "Edit Environment" control panel (e.g., right-click inside an image, or Datamode->Misc, etc.).