I was looking for the USB-C to 3.5mm converter and found it in the box, but there was also another rubber piece that was unexpected. It was nicely packaged and had a product code on it that I could not find in a Google search.
I guess it is to hold the converter to your headphones?
I know people gripe about not having 3.5mm jacks on the newer phones, but with Bluetooth 5 and APTX support, it is becoming less of a problem. The converter jack works fine, and you can buy replacements pretty cheap (<$7). With wireless charging becoming more popular, it might not be long till the USB port goes away. Hopefully this will help get to a waterproof phone. I know you won’t be able to hook up an HDMI cable if the USB port goes away, but with devices like the Chromecast, that is not a big deal either.