I don't have a definitive answer, but:

1. they don't work under Open Transport, and

2. they were presumably designed with early powerbooks and compacts in mind (i.e., models to which ethernet could not be added internally);

3. ergo, I reckon it's a good bet that they'd work under 6.

In fact, I seem to recall that they are unhappy running under anything much past 7.1. They certainly belong to the early System 7 era, so there is probably at least as much backwards as there is forwards compatibility.

You do, however, need the requisite drivers.