Why is rule 1.2 classified as undecidable?

[dg1980]

All you have to do is check the syntax/keywords against the C language standard?
It is even possible with some compilers (e.g. a strict mode)?

[misra-c]

Language extensions may cover more than just extra syntax or keywords. It's legal for an extension to provide a well-defined behaviour for something that is undefined in The Standard. Since detecting some of the undefined behaviours is undecidable it's not possible to decide whether a program is making use of the extension.