Originally Posted by gooselee

I thought some partners had variable award pricing using DL RDM now, also? I could be wrong...but I assumed if this was the case it would especially be true for the "closely held" or JV partners like AF/KL/AM.

In any case, this is correct that the married segment availability is 100% a requirement in order to avoid additive pricing. I have two data points for this in both directions, granted they are both in late 2019/early 2020 so take into account changes since then:
1. SFO-AMS-LHR booked in business/coach, all on KL metal/KL marketed. This did not have an available award through-fare, and thus priced additively at something like 80k one-way. Sure, I'd have loved to have the whole ticket for 70k or whatever but the long trip was in business and the extra 10k for the complete ticket and flight times I wanted was fine. Also on the KL short haul I had a row to myself anyway so it was essentially the same as euro-business.
2. ATL-YYZ-AMS-BGO//AMS-ATL booked in biz/coach mixed-metal (open jaw; we had a bunch of land/boat stuff to to get from BGO>AMS). The AMS-BGO leg was only showing available in economy, and the website would just show unavailable when I tried to search the full route beyond AMS. So when I called to book I was expecting to pay additive pricing which I was fine with. But the agent surprised me by saying, after about a 20-second search, that a mixed-cabin itinerary was available for the outbound and booked the whole thing for 150k RT. The entire phone call took maybe 15 minutes, including seat selection.