2.7 Is there a system variable on 6.0 only, that I can check for its existence with the HPCIGETVAR intrinsic, to say I’m on 6.0 or not?

You can try to execute a benign command such as ":LISJOBQ >$null" introduced in 6.0 and check the value of CIERROR.

You could also use HPVERSION for gross OS-level determinations and, depending upon the way your systems were updated, even the PowerPatch level. For example,

showvar hpversion

HPVERSION = C.55.07 ß MPE/iX 5.5 PowerPatch 7

showvar hpversion

HPVERSION = C.60.01 ß MPE/iX 6.0 PowerPatch 1