Update: the new preferred syntax appears to be:
Some change at google or the SD society site has broken the old approach for filtering searches to SD conference papers, which was
site:www.systemdynamics.org/conferences/* bathtub dynamics
Now it appears that
site:www.systemdynamics.org bathtub dynamics
still works, though with a bit of noise from general SD society pages.
For some reason, the old way still works in a google custom search: