(Not just any such pattern, though -- a still life also means that it doesn't change when you evolve it according to Life rules.)
On the other hand, pseudo-still-life and quasi-still-life arrangements are much easier to construct on average than strict still lifes with the same number of cells.
I think the consensus is that someone could figure out how to construct any given stable 21-bit configuration. The non-strict cases are just a bit too numerous and not interesting enough, so nobody has gone through and formally checked them off the list.