Twitter apparently was working on their own protocol, under a project named BlueSky. I personally feel this was unnecessary and redundant because w3c had put forward activity pub.
However, this project existed just the same. As far as I can tell, the Twitter and blog for the project have not been active since late October and I'm not sure if there's any life to it anymore.
Frontends can just support both. In Mastodon, I can search for either `@username@example.com` or `https://example.com/@username` and both will suffice to show the same result.
That would imply that's an email, when in fact it's just a fedi account.
It's there for a reason, another one (besides accessibility) is to make it easier parsing for other instances.
BlueSky is closer to decentralized SNS, where as AP is actively seeking to become a federated SNS. BlueSky uses DID and independent data repositories, which allows freely migrating b/w instances. AP simply does federation b/w rather classical blog hosting apps.
DID is Decentralized IDentifier. It's a design for decentralized identity registries, independent of central ID providers and authorities. Anyone can verify identity without going through the issuer. This is generally considered as a blockchain tech, though the spec is written in a generic way.
