1) First there were the "hashbang" URLs, that Google said they'd crawl fine, but that did affect Gawker's SEO: http://www.webmonkey.com/2011/02/gawker-learns-the-hard-way-...
2) And instagram uses server rendering for SEO: https://twitter.com/cpojer/status/711729444323332096 (via http://jamesknelson.com/universal-react-youre-doing-it-wrong...)
The "hashbang" URLs (they call them Escaped Fragment URLs) actually required you to use server-side rendering, that had nothing to do with the spider's javascript rendering engine. I used them on a few sites and it worked pretty well, but I have heard of mixed results on sites that weren't originally on hashbang URLs who transitioned to them. They don't recommend you use these anymore.
why not use the RFC terms? https://tools.ietf.org/html/rfc3986#section-3 "url fragment"
Also, it won't help you on server side rendering since the server will never even see it on the request.
Because it's specifically different, on purpose. The RFC doesn't cover the "bang" part....the "!".
>>the server will never even see it on the request
Explained on the (now deprecated) page from Google on this "hashbang" approach:
*"The answer is the URL that is requested by the crawler: the crawler will modify each AJAX URL such as www.example.com/ajax.html#!key=value to temporarily become www.example.com/ajax.html?_escaped_fragment_=key=value"
It will when Googlebot sees the link and rewrites the URL.