The msys2 repositories has mksh, but that is a public domain version of ksh88, there is another version of ksh released in 1993 which is community maintained to this day. That would be ksh93u+m. AT&T stopped maintaining ksh, so the community has decided to add new functionality and squash decades old bugs with a fork.

The upstream github repository which distros like debian or fedora package from, is here. This should be packaged for msys2 because huge amounts of new features that are native only to ksh93 such as compound variables which are C style structs and even some very rudimentary degree of object oriented capabilities. There are a lot of scripts out there that won't work in ksh88 because it uses the features only in 93.

Ksh88 (or rather mksh) should continue to be packaged, but would there be a conflict between ksh93 and ksh88? For a number of unix like systems, ksh is just a symlink to what the actual version it is.