Is your feature request related to a problem? Please describe.
I have no idea what percentage of traffic to domains such as example.org is from Sphinx linkcheck, but I don't think that there is a huge amount of value in any network requests from documentation projects to IETF-standard reserved domains (ref: RFC2606 and RFC6761).

Describe the solution you'd like
I'd like to propose that we report hyperlinks to those domains as ignored. I can't immediately think of a reason to allow this to be overridden -- so perhaps it should not be appended to linkcheck_ignore, but instead an internal hardcoded filter within the Sphinx linkcheck builder code.

Describe alternatives you've considered

• Setting a default value for linkcheck_ignore that includes the IETF reserved domain names.
• Maintaining the status-quo (in other words: do nothing and require that each project add IETF reserved domains to their linkcheck ignore-list explicitly).

Additional context

• Originates from an issue reported at: Option to ignore linkcheck for SSL error #12985 (comment)