Because there is not a post anywhere that I can find addressing this, I’m going to operate as if it’s a bug.
As of this morning (~11:30 am Central time), Soul Splinters are barely dropping from any content, and not at all from Gem Reserve Nightmare Sigils. I’ve tried Infernal Hordes, Helltides, multiple different Nightmare Dungeons including Escalation Sigils and I have looted a total of 13 splinters, 9 of which have come from Whisper caches or Gifts of the Tree.
If this is an intended nerf, it needs a note posted anywhere. It’s also egregious and should be reconsidered.