I disagree with Apple caring about legibility. You can have legibility in simplicity but apple isn't doing that.

Legibility requires contrast, the whole liquid glass transparent background thing kills contrast and thus legibility. Having random noise from the background windows kills legibility.

As for icons specifically - having white semitransparent (vertical gradient) shape on top of brighter color background makes it harder to perceive the shape which is currently only remaining distinguishing feature. Half of them are gray on gray. If you can't recognize the shape while squinting your eyes then it's not a good legible shape for icon. The Messages/FaceTime is other comments mentioned is one of the best/worst examples of this. In theory shape of camera and speech bubble are unique, but in practice the real difference is very small. They are both similar size elongated blobs. Messages have tiny little triangle in bottom left corner, camera has two notches. Overall and in combination with color choice this makes it harder to recognize the shape. Phone also uses the same color but at least the general shape is significantly different.

It's not all completely bad. The rainbow button strip and darker background for Audio Midi setup in my opinion made it more distinct. The contact icon also improved contrast, but only because old one had very bad contrast.

There are plenty of others where contrast was made worse. For example Time machine, Font book, Clock, Finder.