Fundamental flaw here, multiple meanings of the word "seed"even within discussion of just BIP39.
The "entropy" is a better term for the BIP39 starting point. Then the endpoint of the process should be "BIP39 numeric seed".
"Convert the entropy to a BIP39 mnemonic. Use that mnemonic plus a (technically optional but no) passphrase as the inputs to produce the BIP39 numeric seed. End of BIP39. "
is how I would tell the story.
Now, looking at this from *Mastering Bitcoin *
The seed can be encoded using the mnemonic-encode command:
$ bx hd-mnemonic < seed > words
adore repeat vision worst especially veil inch woman cast recall dwell appreciate
The seed can then be decoded using the mnemonic-decode command:
$ bx mnemonic-decode < words
Andreas Antonopoulos is here clearly using "seed" to refer to the entropy. The process described does not involve hashing, or cryptographic "encoding", it's just a change in representation, like going from binary to hex.
Except for the N/32 checksum getting added.
Which means an input entropy of 256 bits is entirely reversible, but the last 8 bits to get to 264 (24 words x 11 bits) is the result of the checksum process.
What the other answers here are referring to is not the "entropy seed" but the output, the "BIP39 numeric seed" which is indeed a one way derivation process.
I believe the
bx mnemonic-decode command may have gotten removed simply because it is useless, or at least I cannot think of a use case.
I do not think it stopped working in the transition to BIP39. EDIT The entropy to mnemonic "conversion" is reversed by many tools - see https://iancoleman.app