I believe this note is in opposition to languages that originated contemporaneously with C but that do not distinguish between upper and lower case. Fortran and Cobol, for example, do not, but also restrict the identifier set to ASCII equivalent letters. 
I think it is clear enough, since it is a Note, but is certainly a candidate for reevaluation in another pass through lex bringing all of this out of 1970s technical lexicon. 

On Thu, Jun 4, 2020 at 6:33 PM Corentin Jabot via SG16 <sg16@lists.isocpp.org> wrote:
LGTM

Nitpick:

[ Note: Upper- and lower-case letters are considered different for all identifiers. – end note ]

Not all characters with case mapping are letters.
I don't know if it matters much if we are going to use better terms in the future
Overall, I'm not sure that the note clarifies anything?

On Fri, 5 Jun 2020 at 00:15, Steve Downey via SG16 <sg16@lists.isocpp.org> wrote:
Posted as Draft at https://isocpp.org/files/papers/D1949R4.html
If there are no objections, I will update to P later this evening and post to the EWG list. 

Diff:
https://github.com/steve-downey/papers/commit/a5a928b12b5664de57d5c0d8a9ca6ba4200d168b
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