As far as I can tell, using filter[availability] does not work.
Cribbing from the Launch UI, I found that I need to use the query param, 'max_availability' with a value of 'private' (or 'public', or 'development' depending on the desired result).
Without the use of the 'max_availability' param, I am seeing development version returned which has a higher version than the extension package with public availability.