Translucent materials in UE4 cannot receive dynamic shadows cast from other objects.
the only exception is for materials using Translucency Shadow Volume which is so short-range and/or broad that basically is only useful for shadows from big objects (i.e. entering a building or a cave).
eyebrows are usually painted onto the character rather than put on top as a translucent mesh. even if you want customizable eyebrows I’d suggest doing it all as different “layers”, all blended into the character material.
eyelashes though, they really need to be translucent. in the stream you mentioned they do say they don’t use the hair shader for eyelashes, but I think it’s not clear if they use a translucent material (and just live with the light/shadow artifacts) or if they use a masked material (and rely on TemporalAA for it to not look like craap)
I’d be interested in knowing what’s the more “official” or at least the more accepted way of doing this.