Is it just an reinforcement bar?
Yes
Is it part of the subframe?
No But indirectly bolts to it.
Is it even needed there?
Yes (Well Nissan seem to think so )
The "W" bar goes across the rear just in front of the wheels and strengthens the chassis, thread a while ago where someone cut a bit out to fit a Zorst
They do rust badly and if that's the part you need
I wondered if they meant the drop links? Best double check