Dead link in IXR introspection server (specUrl points to xmlrpc.usefulinc.com)

[mayur8991]

In the file:

wp-includes/IXR/class-IXR-introspectionserver.php

There is a reference to a specUrl that no longer works:

'specUrl' => '​http://xmlrpc.usefulinc.com/doc/reserved.html',

The domain xmlrpc.usefulinc.com is no longer accessible, so this URL is now a dead link.
As a result, WordPress Core ships with an outdated and broken external reference.

Although XML-RPC is largely deprecated in favor of the REST API, the code still exists in Core, and broken references should be cleaned up.

Steps to Reproduce

Open wp-includes/IXR/class-IXR-introspectionserver.php

Locate the $signatures array where 'specUrl' is defined

Visit ​http://xmlrpc.usefulinc.com/doc/reserved.html

Observe that the URL is not reachable

Expected Behavior

The specUrl should either:

1. Be updated to a valid working reference, or
2. Be removed if no suitable replacement exists.

Environment

WordPress Version: 6.8.2

Also exists in trunk (6.9-alpha)

[panchalhimani711]

@mayur8991

We can use this working reference link here: ​https://xmlrpc.com/spec.md

[mayur8991]

removing the broken specUrl reference—retained only specVersion

[mayur8991]

Added patch

[SergeyBiryukov]

In 60655:

XML-RPC: Update specUrl links in IXR_Server and IXR_IntrospectionServer.

As the original URLs are no longer accessible, this commit uses the latest available copy of the corresponding documentation from the Wayback Machine.

Follow-up to [1346], [56167].

Props mayur8991, panchalhimani711, SergeyBiryukov.
Fixes #63848.