I thought nix did perfect bit-for-bit reproductions at any time on any machine so long as you used flakes to lock the versions of inputs (and also I gather that flakes by default lock out some remaining sources of nondeterminism like reading environment vars). Is that not correct? (I don't see flakes mentioned in that link, nor the linked lobsters discussion, and the only mention in HN comments of the past was someone arguing that it doesn't count because some packages still fail to reproduce, which is notable but would still leave it in at least the same ballpark as ex. Debian)