>You must store and operate on the person's name as is. Requiring a name modified, or modifying it automatically, is unacceptable

But this is simply not true in practice and at times it's just plain wrong in theory too. The in practice part is trivially discoverable in the real world.

As to in theory -- I do in fact want a properly functioning service to use my name in a vocative case (which requires modifying it automatically or having a dictionary of names) in their communications that are sent in my native language. Not doing that is plainly grammatically wrong and borderline impolite. In fact I use services that do it just right. I also don't want to know to specify the correct version myself, as it's trivially derivable through established rules of the languages.

Sure, there are sites that mistreat names in ways you describe, but that does not make it correct.

> I do in fact want a properly functioning service to use my name in a vocative case. ... I also don't want to know to specify the correct version myself, as it's trivially derivable through established rules of the languages.

There would be nothing to discuss if this was trivial.

> Not doing that is plainly grammatically wrong and borderline impolite.

Do you know what's more than borderline impolite? Getting someone's name wrong, or even claiming that their legal name is invalid and thereby making it impossible for them to sign up.

If getting a name right and using a grammatical form are mutually exclusive, there is no argument to be had about which to prioritize.