Suspected System V, and there was definitely a lot on AIX if only because there was CICS for AIX available (maybe still is? Paradoxically it was easier for me to get the mainframe version running... than any AIX system...)
MicroFocus on RHEL is stuff I've seen live recently, including new development, and I think there are still companies buying new releases of COBOL programs.
Similarly, MUMPS is very much alive in financial market, with completely new development being done even if mostly as small component of biggerz usually Java apps
