Oops, I really did not know that. I just took it for granted from what I knew. I'll be looking into that change and when it occurred.
The change occurred sometime before August 1996.
Or at least the policy was being set into place.
There are still a few SP5's, but no-one is being promoted to the rank anymore, and those are are currently Sp5 or Sp6 will likely do PLDC and get Sarge stripes.
The number of Sp5 and 6 has to be pretty insignificant these days though.
The only rank overlap that exists anymore is Sp4 and Corporal.
The difference being they're both E-4, but Corporal is considered a Non-com.
I understand what Tads is saying, there are those in E-5 rank that do not act like an NCO, and there are Sp4's that do.
Hope this narrows your search on the policy change date down.