The lack of audio or tactile feedback when one holds the home button down for an input sentence to Siri is not a "bug" in the classical sense. But it is a "bug" in the conceptual sense. A boatload of people really like and depend on the beep to alert them that Siri is, indeed, listening.
And it's not as if there were situations and/or configurations in which the user thinks they have Siri's attention, but it really isn't listening, even when the audio waveforms are present at the bottom of the screen.In other words, the committee that decided to do away with audio feedback that tells the user Siri is definitely, positively listening for an input command, got it wrong.
What's more, they didn't realize that having the beep – or at least a click/click when the button is down for a Siri command – would be a better idea than their simple solution to do away with it all together.
Which brings up a different idea. Some of the entries in this thread seem to suggest that the haptic click is present when holding the home button down for a Siri input, but my iPhone 6s+ does not do this. Never has. Is that a bug in itself, or is this phone set up wrong?