I recall the old SWUs had some language on the back which said "not valid on certain Delta flights." (Perhaps they should have been called Nearly-System-Wide Upgrades?)This was just as SkyTeam was being announced and AF/DL codeshare was largely in place. I asked at the NYC Grand Central CTO which those certain flights were.

The agent pulled it up and it was only codeshare flights - but I don't remember if it was all the codeshare airlines. South African was on the list; I don't recall if AF was - but my gut feeling is AF wasn't on the exclusions list.

I'd bet some of those rules carry over to mileage and PMUs today.