How long has this technology been on Hive?

Which one? MultiSig? Create new public and private keys to add? Always

How many Private/Public Keys can be created per account?

40, I think, and if you want you can remove or swap any of them at any time, https://multisig.hive-keychain.com

Has anyone implemented this yet?

No one.

To complement this, you can also use Hive Keychain (you need to enable multisig in its settings, and use the real username) to make this multisig request, and the requests appear on https://multisig.hive-keychain.com/#/signRequest