1 Subclause [constraints] describes restrictions on C++ programs that use the facilities of the C++ standard library.
The following subclauses specify constraints on the program's use of namespaces, its use of various reserved names, [...]

2 If a program declares or defines a name in a context where it is reserved, other than as explicitly allowed by [library], its behavior is undefined.

This is specifying library UB.

Sorry, could you please elaborate? My point is: why is defining a macro that is e.g. equal to a language keyword a matter for the library, rather than for the language?

You can change the meaning of including a C++ standard library header.

But the rule is broader than that. A TU that consists entirely of

#define new old

is illegal according to that clause, isn't it? So why can't the rule be moved elsewhere?

(For clarity: I'm not saying to REMOVE that paragraph; I'm asking to move it to a language-specific part, or at leats to leave links from the language-specific parts.)

But the rule is broader than that. A TU that consists entirely of

#define new old

is illegal according to that clause, isn't it?

A TU is not enough. It's "C++ programs that use the facilities of the C++ standard library".

[constraints.overview] says:

Subclause [constraints] describes restrictions on C++ programs that use the facilities of the C++ standard library.

If you don't use the standard library, you can #define keywords to your heart's content (and live with the consequences).

Got it, thank you.

Just wondering: is there a formal definition of "using the standard library" somewhere?

I don't think so.