The big problem with ksh93 is that it is not maintained.

After David Korn retired, an attempt was made to bring out a new version which failed dramatically, and AT&T reverted to Korn's final release.

SmartOS uses ksh93 as root's shell.

Last commit was 11 hours ago, still maintained.

This has some information on the 2020 failure, but it is not what I remember reading.

There were core performance problems in the 2020 effort.

	▲	gaigalas 5 hours ago \| parent [-]
		ksh2020 is `v-` based, the repo I mentioned is an `u+` fork with backports from the failed attempt. Most notably, it doesn't change the build system. It's what Debian and Fedora use, so AT&T doesn't matter anymore. Real-world upstream is the repo I linked, which is maintained. All the 2020 drama is now obsolete.