How? The usage was very easy. You select a contact and add them as your recovery contact (by selecting contact from your contact list) The system adds the key in the background. If they don't have the app, the app asks you to tell them to install the app (viral growth?). The users didn't need to know any thing technical. But install app, and click yes/no like they do with a 2FA app.

I think the challenge is more coordinating the 8 people who will be a trusted part of your life long-term. Also they’d have to be sure to keep their fragments of the key intact through replacing devices, etc, no? Seems like just keeping a Yubikey in a safe deposit box would be simpler.

if they replaced their device, their new device would still preserve your key, just like you replacing your device keeps your key.

Define ‘safe deposit box’?

If it is a safe at your home, you need to have a fixed home address in the first place, and the usual advice about off-site backups also applies.

If it is at friends or family, you’re back at the same problem.

If it is a rented deposit box, you need to trust the company you rent it from (banks don’t usually offer such services anymore, and there are risks like in [1])

[1] https://www.nytimes.com/2019/07/19/business/safe-deposit-box... (archive: https://archive.is/7qbkR)

I don't have eight people, what then?

You use different numbers, for example 3/5 or 2/3.

You have to have at least 3 peers, though (IIRC, 2/3 is the minimum split possible that would provide fault tolerance).