It caught my attention that the specification supports only P2PKH addresses. Meanwhile, it appears that some implementers of the standard make it possible to derive other address types (native segwit primarily) and that that information is communicated through out-of-band channels.
For instance, the biggest payment code directory (https://paynym.is) communicates that information by verbally stating “Derives native segwit addresses (p2wpkh) as well as legacy addresses”.
The proposed change to the BIP would address the issue by allocating several bits to communicating the exact address type that the recipient is deriving. This removes potential confusion, funds being sent to incorrect addresses, and makes the standard more self contained. The BIP is still in a draft state so modifying this bit would benefit everyone using the standard.
Backward compatibility with the earlier iteration of the standard is locked in with the provision stating that it is acceptable to send to other address types, as long as that information is communicated elsewhere (as is currently the case with the Paynym directory).
Tagging @SamouraiDev and @justusranvier.