Where does it list the preferred alternatives to banned features?

For example:

> The <filesystem> header, which does not have sufficient support for testing, and suffers from inherent security vulnerabilities.

For most of the banned library features, the preferred alternative is listed right there in the notes. <filesystem> is one of the exceptions.

base/files

Gonna venture a guess and say probably https://www.chromium.org/developers, as that's where all the information for folks who actually need to know that kind of thing lives.