Basic completion in ksh is as easy as defining an array. From https://man.openbsd.org/ksh :
Custom completions may be configured by creating an array named ‘complete_command’, optionally suffixed with an argument number to complete only for a single argument. So defining an array named ‘complete_kill’ provides possible completions for any argument to the kill(1) command, but ‘complete_kill_1’ only completes the first argument. For example, the following command makes ksh offer a selection of signal names for the first argument to kill(1):
set -A complete_kill_1 -- -9 -HUP -INFO -KILL -TERM
Is this in the Korn & Bolsy ksh88 book?
Or is this ksh93 syntax that oksh back ported?
I don't know actually. Are there currently used version of ksh that are not derived from ksh93?
Very much so - pdksh was a bsd-licensed clone that is the ancestor of mksh (Android's system shell) and OpenBSD's oksh, which you previously mentioned.
David Korn thanked the pdksh authors and maintainers for making it available in the years when true ksh was closed (ksh88) or open but licensed with awkward terms (ksh93 for several years).
David Korn interview (I asked one of the questions):
https://m.slashdot.org/story/16351
"First of all pdksh is a ksh88 clone; and I might add a better clone than the MKS Korn Shell...
"I don't know the pdksh development team but I would like to thank them for the service they have done in making a version of ksh available while ksh was proprietary. I have noticed remarkable improvements in pdksh in its ability to mimic ksh88 functionality. I don't know what plans the pdksh development team has now that ksh93 is available in open source form, but I certainly would help them try to maintain compatibility if they do continue pdksh distribution. Otherwise, I would hope that they would pick up the ksh93 source and help support and enhance it."