Is this option still available?
I just tried it in my gp installation – and customize-> site identy only shows (text copy+paste):
Site Title
Hide site title
Tagline
Hide site tagline
Logo
Logo Width
px
Site Icon
Site Icons are what you see in browser tabs, bookmark bars, and within the WordPress mobile apps. Upload one here!
Site Icons should be square and at least 512 ร 512 pixels.
So basically like described in the documentation – but no retina option there.
I am using current gp childtheme on current wp in a virtual machine.