mrdoob/three.js — comment on commit dda3846a5f23bd2e214374aa1fe49b1f5eefc534
File:  line 0
By alteredq:

Yup, this was used just for stencil shadows.

Then @zz85 also used edges in `SubdivisionModifier` but these seem to be separate implementation, just reusing some pieces from the old code.