The most recent pigment to effectively vanish is Quinacridone Gold as the sole remaining source of this pigment (PO 49) was Daniel Smith. By chance, I happened upon it while doing some Amazon shopping (as I always do) and really liked the colour swatches I saw. So, I picked up a 15ml tube of it and it turns out that I was buying one of the last batches to be made using a single pigment. Newer formulas from Daniel Smith are similar to other paint makers as it now uses PO 48 (Quinacridone Burnt Orange) and PY 150 (Nickle Azo Yellow).
So, what happened? Well, PO 49 usage was basically discontinued by the automotive industry back in 2001. Daniel Smith was apparently paying attention and so bought up all the remaining stock of the pigment it could find and continued to make paints with it, notably using it for Quinacridone Gold and Sap Green. Well, they've run out. It was, really, kind of inevitable, but having just discovered this colour and how vibrant it is, it makes me rather sad. Sad enough that I quickly bought two more 15ml tubes of it! Based on my rate of usage, that should last me a very long time.
Now, I had never used the original two pigment Sap Green which had used PO 49 and PG 7, but some swatches of that showed some really gorgeous colour that I don't see from current versions, despite the alternatives being pretty good on their own. So, if you do have some true Quinacridone Gold and you have some Daniel Smith Phthalo Green (BS) in your possession, then you can make their original Sap Green. Denise Soden has a video that demonstrates (jump to 8:30 to see her mix):
Anyways, maybe we'll get lucky and PO 49 will once again become interesting to the automotive industry.