AIX defaults to ksh88. My first job out of college a long time ago was to maintain and enhance AIX's various installers, which involves some 100KLOC (!!) of ksh scripts. I was one of the first people on the team to move newer projects to Perl- this was early 00's and somewhat fashionable back then, and the only other scripting language supported on AIX at the time. In retrospect, it's possible everything might have been better off left as ksh :-)
Commercial UNIXen that bundled CDE had two ksh versions - ksh88 installed as the POSIX shell, and ksh93 installed as the CDE graphical shell as /usr/dt/bin/dtksh.
The dtksh binary bundled Motif X/Windows support, and scripts could easily and quickly build complex GUI applications. This actually remains the fastest and least resource intensive way to build a quick GUI.
The AIX installp format is my favorite package manager. It was fairly easy to debug it until it hit the ODM.
The only time when AIX utility debugging got weird was when multibos went off the rails.
I started using ksh on AIX and it has been my favorite shell since then. I've written a few thousand SLOC worth of utilities and customization scripts in KornShell.