Elemental Overflow is one of the currencies added with Dragonflight, used for Primal Invasion armors, and being expanded to purchasing Forbidden Reach items in Patch 10.0.7.
As of note, players have shown concern for pricing on items in Patch 10.0.7, such as the Noble Bruffalon needing a whopping 0 Elemental Overflow, but in good news, Forbidden Reach is quite literally overflowing with Elemental Overflow!