It’s built in to Safari, Chrome, Edge and other browsers, apps can easily integrate with system libraries for Windows Hello or Touch ID as they would anyway.
As for estate planning, set up a spare key that you can keep at a relative’s place, or add others’ accounts to your “Family” in Google/Microsoft/Apple/etc. Either they have their own keys and the company is aware of the handover or they have a copy of yours — such as you logging in on their device or keeping a FIDO2 key at their house and they can pretend to be you. A service like 1Password Family could also be of use here.
As for estate planning, set up a spare key that you can keep at a relative’s place, or add others’ accounts to your “Family” in Google/Microsoft/Apple/etc. Either they have their own keys and the company is aware of the handover or they have a copy of yours — such as you logging in on their device or keeping a FIDO2 key at their house and they can pretend to be you. A service like 1Password Family could also be of use here.