I find myself agreeing with the rejection. As much as I dislike the ridiculous amount of emojis in Unicode, and their increasingly widespread use, they do fit plain text communication, whereas an "external link" symbol does not. My heuristic is something I'll call SMS sniff test : would it make sense to type that symbol into a text message? If it wouldn't, then it doesn't belong in Unicode. "External link" symbol tel…
But the question is not if the icon is practical or silly, the question is if it makes sense as a Unicode character or should be handled at some other level like a GUI control or widget.
Should GUI icons, widgets and controls be covered by Unicode? That is a can of worms.
I understand the usefulness of the "external link" icon, but should it really be something you arbitrarily type into a text? Shouldn't it be part of the rendering of a link?