Drop "always considered empty", and also ", e.g.: snapshot objects with many branches".
The latter reads awkwardly, the former is unclear at this stage.

You can then add at the end something like: "note: the archive directory cannot be listed with ls, but entries in it can be accessed (e.g., using cat or cd)".

(In passing: the duplication of these snippets between the design doc and the docstrings is annoying, but I don't have a better solution to propose for now.)

aren't we gonna need this later on, e.g., to populate the forthcoming cache/ dir?
if yes, better to keep it around and just not using it for now
(if not it can go of course)

uniform this with the changes suggested above

please add a JSON_SUFFIX = ".json" constant/attribute somewhere and use it here to avoid repeating ".json" thrice

good riddance meta/ dir, so long and thanks for all the fish

minor: I think we've consistently used the archive/<SWHID>.json meta-syntax elsewhere, please do the same here

ah, revenants of old code reviews :-)

rather, something like: assert {"archive", "origin"} <= set(os.listdir(fuse_mntdir))

(which just checks that the entry you'd like to see are present; and it's also order-independent)