Thank you, I was unaware of this. I found the relevant section in the doc that was linked from your original link:
2. TLDs for Testing, & Documentation Examples
To safely satisfy these needs, four domain names are reserved as listed and described below.
.test
.example
.invalid
.localhost
* ".example" is recommended for use in documentation or as examples.
* ".invalid" is intended for use in online construction of domain names that are sure to be invalid and which it is obvious at a glance are invalid.
* The ".localhost" TLD has traditionally been statically defined in host DNS implementations as having an A record pointing to the loop back IP address and is reserved for such use. Any other use would conflict with widely deployed code which assumes this use.
https://tools.ietf.org/html/rfc2606#section-2
3. Reserved Example Second Level Domain Names
* example.com
* example.net
* example.org