https://github.com/cplusplus/draft/issues/9228
>>>>>
In the current standard, we have four references to

Unicode Standard, Chapter 3.9 \ucode{fffd} Substitution in Conversion
Something is wrong here.

There is a Chapter 5.22 since Unicode 11 which is titled U+FFFD
Substitution in Conversion, but it's not Chapter 3.9. This chapter number
and title remain unchanged up to and including the latest Unicode 18 draft.
That Chapter 5.22 used to be called "Best Practice for U+FFFD Substitution"
for several standards before Unicode 11, so it's not just that it's been
moved from 3.9 to 5.22.
Chapter 5.22 makes several references to Chapter 3.9 Unicode Encoding Forms
(which also includes the definition of "maximal subpart"), but Chapter 3.9
has never been called "U+FFFD Substitution in Conversion".
Basically, we need to decide for the existing references whether they are
intended to refer to §3.9 or to §5.22 and how to phrase that.
>>>>>

Where should this point to? The Unicode standard does not have "stable
names" so we need to pick a version to pin to for reference.

Which Standard and which section?