That's why three different procedures for docs?

Python docs procedures: (0) Devguide, (1) PEPs w/ front matter in RST, (2) RST in /Doc with Sphinx, (3) MD and TXT in /InternalDocs without a toctree

The .. warning: or even admonition directives could be used for indicating that docs under /internals are not public API and can change with or without a PEP; though that should also or at least be indicated in the source unless that's a given expectation that not marked public APIs are not to be considered stable

How many, many times has a project said, "Don't use, internal only", only for it to become an industry-wide common "trick"?

Saying "here be dragons" is not enough to discourage people whose job it is to be creative.