As the author pointed out, emojis seem to clearly violate this excuse. What possible justification do they give for emojis? There is no way [I originally inserted the poop emoji here, but it was stripped out. That kind of reinforces my point.] is an element of plain text.
So there's a situation of dual standards - all the weird characters that were included in any pre-Unicode text encodings for whatever arbitrary reasons are in Unicode and are always going to be there; but all the new weird characters need appropriate justification for inclusion and are likely to be denied.
Nobody would type an external link symbol. It would be added by the UI presentation layer. It is not part of plain text, because it is not typed as part of textual content. But the poop emoji is.