Not sure in the case of Hammerhead (I've got a Wahoo and a Garmin), but mostly You can opt out of updating Your head unit. Problem is, it's an all-or-nothing. By not updating You also lose whatever bugfixes will be coming in the future updates.
That being said, this week's update that removed Di2 also added some nice location-based auto lap features, which are way more useful than an icon telling me which gear I'm in, which I already know, so I updated.