One historical use in the 1400s doesn't merit a character and never did
One known and surviving use. It is possible that it exists in other places, since the vast majority of the planet's written work has not been digitized. It may also have been used other places that have not survived.
Just because it's not important to you does not mean it is not important.
The fact that is survived for 600 years makes it interesting and worth saving. It is infinitely unlikely that anything you do, write, or say will last that long.
Sure it's possible, but there should be a higher bar than "it's possible it's used more than once" for meriting inclusion in the standard keyboard of billions of devices worldwide.
The thing is, looking at the page, there are many other characters that were not added - the large red С-looking characters, for example. But for some "bizarre" reason, those were not included in Unicode...
Of course, the simple answer is that Unicode actually includes any character that someone cares enough to ask to be added, with rare exceptions.
One known and surviving use. It is possible that it exists in other places, since the vast majority of the planet's written work has not been digitized. It may also have been used other places that have not survived.
Just because it's not important to you does not mean it is not important.
The fact that is survived for 600 years makes it interesting and worth saving. It is infinitely unlikely that anything you do, write, or say will last that long.