Hm, now that you mention it, I always thought of the external link symbol as being a box with an arrow coming from inside it and protruding out of the upper right hand corner, but I don't see that symbol anywhere in Unicode, and I'm not sure why I have that association.
There is the U+1F517 link symbol but I'm not sure that's communicating the same thing.
There is the U+1F517 link symbol but I'm not sure that's communicating the same thing.