Why are non-hd wallets needed for #9662?

Because the goal of #9662 is to not have the internal wallet generating ANY keys. It's to watch specific addresses only.

Please no. We really don't need a combinatoric blowup in the number of supported wallet options and setups. How about instead support having a public-only HD seed (and if none is provided generate one, who cares, you're just watching a few extra addresses that will never be used).

@TheBlueMatt That assumes all wallets are HD. Additionally, using public derivation is contentious. Finally, watching addresses not part of the real wallet has risks - someone could gain read access to your wallet and send to those addresses, and you'd mistakenly think they were safe on your hardware wallet.

All newly-created wallets will be HD, though, and supporting a matrix of HD, Segwit, Watch-Only, etc, etc and all combinations thereof blows up maintinence complexity a ton. In any case, a watch-only wallet is going to need a mode that forces it to never generate new addresses, so having an HD seed or not having an HD seed wont matter - you still wont get new addresses.

Since #9662 should be disabling all forms of key generation, I don't see why you need to have -usehd=0. Why can't you just have a wallet type which has no private keys whatsoever? Instead of doing -usehd=0, you just have -disablehot which then doesn't generate a HD master key or any other key.

@TheBlueMatt Watchonly-only wallets aren't HD. Maintenance complexity isn't affected, because we still need to support non-HD wallets regardless. I see no value in writing an ignored HD seed - it only creates the risk that we accidentally use it rather than crash or fail.

@achow101 I suppose that's a possibility, but I don't see the harm in having the option...

From a user perspective, if we don't support creating new non-HD wallets, then it makes no sense to provide a -usehd=0 option.

Agree with @achow101. There is really no need to set hd/nohd when key generation is turned off completely.

Closing for now.