http://www.techradar.com/news/the-touch-controls-are-heading-back-to-your-google-home-mini

Back in the old days, you used to be able to touch the top of the Google Home Mini to speak out your commands or questions to it, and to start and stop music and handle other features, but the functionality was pulled after some of the devices were found to be locked in a persistent spying mode.

You've always been able to activate the speaker with an "OK Google" command but the touch feature got disabled in October. Two months later, Google apparently thinks it's safe to bring the physical controls back to the Google Home Mini, as the functionality is included in the latest preview for the firmware of the device.