Leaving the plug out of the later series engines might have been a cost cutting measure, eliminating a part plus a machining and assembly operation. Kawasaki must have come to the conclusion that having a restriction in that circuit was good enough for the correct operation of the oil cooler.
Its probably not as "efficient" as having the plug in there but I don't think its a cause for concern.
The GPZ pans with the oil level sensor always seem to leak, mine leaked even after changing the O ring. I made a plug for it and had it welded shut.
Recently I converted a 650 oil pan to run a oil cooler from that front oil feed galley.
650 and 750 Spectre (sensor hole not machined)
650 modified (custom diverter plug)
GPZ750 with sensor plug welded in.
On the later oil pans they left all the cast features in but eliminated the machining. The Spectre does not use a oil cooler so there are no machined holes to attach the fittings (no diverter plug)
I plan to run a much larger oil cooler than a stock Kawasaki unit so I opted to run a diverter plug emulating the GPZ pan.