Emoji week continues on Daring Fireball. Glenn Fleishman, writing for Macworld:
As part of this update, Apple has also redrawn most faces and
figures, among other changes. For the quasi-figurative
smiley-style emoji, the changes are very slight. In most cases,
you need to zoom way in to see them, although the subtle effects
remain noticeable on a Retina or 4K display. Smileys now have a
slightly different light source, with a more intense “hottest”
point at the top, some lines are thinner, and the 3D effect is
slightly more pronounced.
The way I’d describe it is that the old Pac-Man-style faces had an Aqua-like high-gloss texture. The new ones are no longer glossy.