A much larger base for ksh (as a pdksh descendent) is Android. OpenBSD is a tiny community in comparison, although Android has acquired code directly from OpenBSD, notably the C library.

The vi editing mode is always present in ksh, but is optional in dash. If present, the POSIX standard requires that "set -o vi" enable this mode, although other methods to enable it are not prohibited (such as inputrc for bash/readline), and as such is a "universal trick."

The article is relying on some Emacs mode, which is not POSIX.

$_ is not POSIX if I remember correctly.

History in vi mode is easier, just escape, then forward slash (or question mark) and the search term (regex?), then either "n" or "N" to search the direction or its reverse.

I've seen a lot of people who don't like vi mode, but its presence is the most deeply standardized.