For 32 px and above, vector images are a valid choice.
Now we just need a file format that combines all this. :)
Well, that depends on your screen's density and how much you care that your design is aligned to the pixel grid. For a very long time most iOS icons were hand-aligned to the pixel grid, and when they switched to vector icons in iOS 13 they looked awful because they were aligned poorly.
TTF?
I understand what they're trying to do with including the icon as metadata, but since they control the filesystem format as well, couldn't they just increase the storage available so they could store an extra kb or so? I'd expect that on modern hardware, reducing the number of reads by stuffing bytes into odd corners of each file's metadata is far less effective than just arranging a block of data to be read all at once.