It refers to a new type of computing elasticity proposed in the Boxer: FaaSt Ephemeral Elasticity for Off-the-Shelf Cloud Applications paper.
Ephemeral elasticity is running applications across both VM/containers as well as FaaS with the goal of having a single orchestrated application deployment that takes advantage of both types of infrastructure.
Thus, ephemeral elasticity is proposed for affordable and highly elastic cloud infrastructures.
Why it is not available today?
Currently, FaaS and VMs/containers operate under two different models.