>>>> I do not know the history of replacing an undisplayable
>>>> character with a diamond. The U+FFFD Replacement
>>>> Character glyph looks similar, but the cause/consequence
>>>> link might be in either direction.
>>>
>>> Can't we check this with the Emacs source if that's where
>>> the change happened?
>>
>> No, because the replacement with diamond happens in the
>> Linux console no matter if Emacs or something else
>> is running.
>>
>>     bash$ echo $'\U0001F4A9'
>>     ♦
>
> ???

https://dataswamp.org/~incal/char/emacs-char.png
https://dataswamp.org/~incal/char/tty-char.png