I was notified by one of the developers of mksh that the SPDX license field used in our mksh package was incorrect, and that the details can be found here: http://www.mirbsd.org/TaC-mksh.txt
In particular, most of the code is released under the MirOS License, the expr.c file additionally contains code under the Unicode-DFS-2016 license, and a copy of strlcpy (not used, libc's version is used instead on Adelie) is included under the ISC license, as well as a copy of the printf command-line utility (not built) under the 3-Clause BSD license.
There is also a Windows .ico file released under terms of Appendix D of the document that I've listed, but just like strlcpy and printf, it's not used by the build system or included in the resulting .apk file.
This means that the correct SPDX for the shell, as built on Adelie, is: MirOS AND Unicode-DFS-2016
This MR corrects the license field in the APKBUILD, as well as updating it to the newest version (in separate commits).