Heh, not necessarily, that's true, in the sense that all the possible arrangements haven't been tested and shown to be constructible.
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.
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.