HTTP中，什么是"X-" Prefix header?
例如 github API 的response headers有很多X前綴的頭部:
查一下MDN文檔:
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.
原來是自定義頭部。但是上面文檔說，已經deprecated，過時了。
作用
和HTTP headers一樣，都是提供額外的信息。
Their main purpose is for providing additional information
RFC表明X前綴的自定義頭已經過時了，代替解決方案是什么？
根據keycdn，推薦繼續保留:
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.
根據stackoverflow，推薦繼續保留：
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.
總結
雖然RFC已經不推薦使用帶X前綴的http 頭部，但是不推薦不代表禁止。根據stackoverflow和google一些資料，推薦繼續使用這樣的自定義頭部，有時候會提供極大的便利。經過本人觀察，很多網站還在使用帶X前綴的HTTP頭部。