Discussion on: Where to Put Response Metadata - Envelope or HTTP Headers?

RFC 6648: Deprecating the "X-" Prefix and Similar Constructs in Application Protocols

The deprecation of X- is mainly concerning possible standardization of these items. But:

When it is extremely unlikely that some parameters will ever be standardized. In this case, implementation-specific and private-use parameters could at least incorporate the organization's name (e.g., "ExampleInc-foo" or, consistent with [RFC4288], "VND.ExampleInc.foo") or primary domain name (e.g., "com.example.foo" or a Uniform Resource Identifier [RFC3986] such as "example.com/foo"). In rare cases, truly experimental parameters could be given meaningless names such as nonsense words, the output of a hash function, or Universally Unique Identifiers (UUIDs) [RFC4122].

I do think headers are the place to put this information, it always has. There is just the rather time consuming part of figuring out the proper header usage. To simply put an X- in front of the header name is a bad practice (see above RFC for the reasoning.)

What you need to do

1. Find an existing, or work in progress, standard; and use it.
2. Think really really hard if your custom entry is standard worthy.
3. Only then use a vendor prefix. e.g. DevTo-Gizmo

As for the example of this article, there is a standard RFC 5988 to provide referential information in the HTTP headers. Note that they are working on RFC 8288 which will replace it. So it is probably best to follow RFC 8288. (And regularly check back for changes).