BIP 93 (aka codex32) is a pretty cool way of backing up a Bitcoin private key: you split the key into a number of different parts using Shamir Secret Sharing, but you only need a threshold number of those parts to reconstruct the whole key.

This gives you some of the benefit of multisig without the onchain footprint. LIke multisig, you can disperse the pieces needed to sign a transaction.

Like multisig some number of these pieces can be lost or compromised without loss (as long as it is less than the threshold needed to reconstruct the key).

Unlike multisig, you still need all the keys in one place to sign a transaction (I'm pretty sure). With a traditional multisig wallet, you could sign a transaction with each key separately, whereas with BIP 93, you need to bring all the parts together to recreate the whole key and then sign.

While awesome, not too many wallets support BIP 85 (although there is a draft PR for adding it to Core.

BIP 85 is also pretty cool. It describes how you can use one master seed to derive seed phrases for a number of different wallets. These derived seed phrases can be recreated at any time by your master seed, but if one of them leaks, it does not compromise your master seed.

For instance, if you have a master seed that you use for your cold storage, you can use BIP 85 to derive a new seed that could be the seed you use for a lightning wallet on your mobile phone. If your phone gets compromised, you might lose the sats in your lightning wallet, but you won't lose your cold storage. If, however, you lose your phone and you need to restore your lightning wallet, you can use your master seed to regenerate the seed you used for your lightning wallet.

BIP 85 is currently implemented by BItBox, ColdCord, and Jade. Not as many software wallets seem to support BIP 85.

Ben Westgate (who is awesome) is proposing adding BIP 93 as an application of BIP 85.

I'll admit, this makes my head a little fuzzy, but I think what he's proposing is that you could use your master seed to create a set of key shares which, when put together, can be the key to a new wallet. If this key is compromised, it still doesn't compromise your master seed.

I'm unclear why one would need a "deterministic" way to generate key shares, but I generally find the stuff Ben Westgate works on to be interesting, so I'm looking in to this.