Why wouldn't it be part of plain text? People type it as part of their text messages constantly. They certainly use it as a part of plain text.
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.
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.