I confirmed with another developer in addition to my own experience, that this option does not work. All of my other style options do, though.
Sample usage:
const styleSet = window.WebChat.createStyleSet({
bubbleBackground: 'rgba(252, 229, 53, 1)',
bubbleFromUserBackground: 'rgba(4, 234, 104, 1)',
paddingRegular: 10,
sendBoxHeight: 50,
bubbleMinWidth: 400,
bubbleMaxWidth: 700,
microphoneButtonColorOnDictate: '#FF00FF',
hideUploadButton: true
});
I confirm also that hideUploadButton does not work.
However, the button may be hidden in the following way :
import { createStyleSet } from 'botframework-webchat';
const styleSet = createStyleSet({
});
styleSet.uploadButton = { ...styleSet.uploadButton,
display: 'none'
};
...
render() {
return (
<ReactWebChat styleSet={styleSet} directLine={ this.directLine } userID="" />);
}
@wiazur and @B7A9F hideUploadButton: true is working fine for me (screenshot below). Are you using production build - /latest/webchat.js ? . It may not be in production build yet. Try using dev build - /master/webchat.js

Thanks @xenflux that worked! It got rid of my other webchat formatting, but this will be great until it reaches production. Do you know when that might be? I can share this with my team. Thanks.
@wiazur Great! Glad that helped. Regarding when it'll be in production, that would be something @compulim or @corinagum can answer.
This is now released in /latest/! Thanks for the help @xenflux