According to the internal RTS spec on GitHub here...
https://github.com/sphere-group/sphere-doc/blob/master/internal/tileset.rts.txt

...the obstruction data area is apparently obsolete.  Is this true?  If so I can avoid loading the obstruction section entirely, but then how are obstructed tiles specified now?

Then again, the RMP documentation says the tileset filename string is obsolete also, when it's clearly not (Sphere Studio writes this string and the engine rightly honors it), so... I'm confused.

I think that the docs are just wrong when they say `OBSOLETE'.

So I still have to load the obstruction data, then.  Makes sense.  I think I'm going to stop relying on the format docs and start looking at the Sphere 1.x (or Sphere Studio) source to figure out the files.  Going by the docs alone is going to get me in trouble it looks like.