Ok, but what do you mean by a "still life" of 21 cells? Is that a rectangle in a larger field or is it an arbitrary set of cells?

If you want to see what these 21-bit still lifes look like, their glider construction recipes are all stored online, on Catagolue (no, that's not a misspelling):

https://catagolue.appspot.com/census/b3s23/synthesis-costs/x...

A pattern with 21 live cells in any bounding box.

(Not just any such pattern, though -- a still life also means that it doesn't change when you evolve it according to Life rules.)

Also we can only do the strict still lives (i.e. those which are connected https://conwaylife.com/wiki/Still_life#Strict_still_lifes). We can't necessarily put down a 10-cell still life and an 11-cell still life near to each other.

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.

Thanks for answering!