Does anyone know about the prehistory why it is called "unix98 ptys"?
   I have discussed it with Soda-san, but we could not come to a conclusion.
   (I suspect it is the Linux dialect.)

I was somewhat actively involved with adding /dev/ptmx to Linux.  At
that point Unix98 was the name for the SUS of the day, before it
became POSIX 2001, so these became the Unix98 pty's (as opposed to
BSD-style pty's).

Mark