for some reason I'm annoyed by all the @ symbols in these new things

Yah, the one thing that I like about bluesky is their name-spacing: “sports.npr.org”

We are coming full circle back to Usenet.

it was annoying to me to type it too! There are also webfinger URLs that serve the same purpose like https://mastodon.social/@gargron == @[email protected]

That's not a WebFinger URL. WebFinger URLs point to /.well-known/ path that returns a JSON document with links to other places for the user. To simplify WebFinger use by a human, we introduced acct:[email protected] syntax. And (at least my) WebFinger clients can assume the acct: part, much like email clients do for the email: scheme.

I like the Matrix format, "@user:domain.tld"

I prefer the email format: [email protected]. There's no reason that can't be used for social media.

what's the reason for the initial @ symbol, seems superfluous

So it doesn't get mistaken for an email address.

It's on Mastodon though, the automatic linking wouldn't add mailto: at the front, think it's worth the cost of a few people not being able to post their email address without a mailto:

Referencing one of these username handles in external CMS systems (news sites, blogs, etc) might automatically convert them in Rich Text editors, or just Word processors, or even operating system toolings. Seems worth not re-using the same URI format.

I would guess so that people don't confuse it with an email address

It originated organically on Twitter as early users wrote “at” each other. Following that Twitter formally adopted it, and Mastodon imitated Twitter.

https://en.wikipedia.org/wiki/Mention_(blogging)#@_(at_sign)

Within Mastodon’s UI usernames are typically displayed as just e.g. @breaking, avoiding the visual noise of multiple at signs.