The actual definition of undefined behavior given by § 3.4.3 is: "behavior, upon use of a nonportable or erroneous program construct or of erroneous data, for which this International Standard imposes no requirements."
Since no requirement is imposed, compilers can do as they want.
I am not quoting from a footnote.
Footnote 1 pertains to § 1.
Footnote 2 pertains to § 3.19.5.
There are no footnotes pertaining to § 3.4.3.
And do you have a citation for the proposition that footnotes or notes of any other kind are not normative?
This is a general principle of ISO standards, see https://share.ansi.org/Shared%20Documents/Standards%20Activi...:
> 6.5 Other informative elements
> 6.5.1 Notes and examples integrated in the text
> Notes and examples integrated in the text of a document shall only be used for giving additional information intended to assist the understanding or use of the document. They shall not contain requirements ("shall"; see 3.3.1 and Table H.1) or any information considered indispensable for the use of the document, e.g. instructions (imperative; see Table H.1), recommendations ("should"; see 3.3.2 and Table H.2) or permission ("may"; see Table H.3). Notes may be written as a statement of fact.
Besides, clang does the same thing with -std=c89, where this provision was emphatically not a note.