Computer Network - header HTTP with the prefix X

In HTTP, what is the X-prefix header?

For example github API 's response headers have a lot of X prefix head:

check MDN documentation :

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