Apple gave LastResort to Unicode Consortium back in 2008[1].

Big Sur currently shipped with version 13.0, so presumably it's the same font (version info obtainable by running `system_profiler SPFontsDataType`).

[1]: https://www.unicode.org/mail-arch/unicode-ml/y2008-m06/0019....

I'm pretty sure it's the exact same. Both of them use what looks like Chicago (!) as the typeface for the text in the edges.

As per the README from your link: "The Last Resort glyphs were drawn by Apple Inc. and Michael Everson of Evertype."

There is also Unifont, but it is quite a bit different to last-resort-font:

https://unifoundry.com/unifont/

The article appears to be from 2002.