This is common on almost all modern chainrings except those for single speed, or intended for the innermost position of doubles and triples. It's a shifting gate allowing the chain to move on and off the ring easier during shifts. You don't need it on the innermost sprocket because the chain is always lifting up from those, and never coming from a smaller neighbor. You'll notice similar cutouts, and/or thinning of certain teeth on each sprocket of any modern cassette.
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