HTTP中，什么是"X-" Prefix header?
例如 github API 的response headers有很多X前缀的头部:
查一下MDN文档:
Custom proprietary headers have historically been used with an X- prefix, but this convention was deprecated in June 2012 because of the inconveniences it caused when nonstandard fields became standard in RFC 6648; others are listed in an IANA registry, whose original content was defined in RFC 4229. IANA also maintains a registry of proposed new HTTP headers.
原来是自定义头部。但是上面文档说，已经deprecated，过时了。
作用
和HTTP headers一样，都是提供额外的信息。
Their main purpose is for providing additional information
RFC表明X前缀的自定义头已经过时了，代替解决方案是什么？
根据keycdn，推荐继续保留:
Although the recommendation to use X- has been deprecated, it doesn’t mean that it is no longer supported. In fact, there are still many scenarios where X- continues to be used.
根据stackoverflow，推荐继续保留：
Just as there are many kids that will never end up as professional athletes, many custom headers will never end up as standards. I'm inclined to keep the "X-" on those
If you're using custom HTTP Headers (as a sometimes-appropriate alternative to cookies) within your app to pass data to/from your server, and these headers are, explicitly, NOT intended ever to be used outside the context of your application, name-spacing them with an "X-" or "X-FOO-" prefix is a reasonable, and common, convention.
总结
虽然RFC已经不推荐使用带X前缀的http 头部，但是不推荐不代表禁止。根据stackoverflow和google一些资料，推荐继续使用这样的自定义头部，有时候会提供极大的便利。经过本人观察，很多网站还在使用带X前缀的HTTP头部。