As far as I am aware, this is a new feature in 4.9 (for mobile devices)
See the 4.9 release notes here:
Under the section “NEW: DYNAMIC CHARACTER SHADOWS FOR MOBILE”
I am referring specifically to the section that says “To enable this feature, turn on the “Cast Modulated Shadows” option on a directional light!”
I have added a “mobile” tag to this question to make sure that people are aware that this is a mobile specific question