But you may also say "Next to the link you will see [EXTERNAL LINK] which indicates that the link leads to another website."

I guess that is a good point, but I guess that the backspace symbol gets a bit of a free ride by literally being on your keyboard.

That reasoning would make every conceivable icon a valid target for inclusion.

To actually get the character included, you can't just reason that it might be used in a certain way, you will have to demonstrate it with actual and authentic use in the wild. The accepted proposal for the inclusion of the power symbol is a nice example of how this works.

If you can find a bunch of printed manuals or books or websites that actually use this icon in this manner you might be able to submit a successful proposal.

The web is full of those symbols, so I'm sure there are books (or online resources) explaining them. And if not, someone should do something about that.

I'm not opposed to the backspace symbol, but by that logic you could include anything in unicode as anything could a symbol you'd like to refer to.

I mean, you might want to say "External links can be identified by their □ marker" or "The Windows key is marked □ and can be found to the left of the space bar"