This doesn't leave Ice Cream Sandwich users completely high and dry, but it will prompt some serious decisions. Many developers will have to either deliver at least two versions of their app (with and without legacy support) or else accept that they're losing a portion of their potential audience, however small it might be. It effectively amounts to the end of support for Android 4.0, even if the software remain useful on some level for a while to come.
Google isn't too worried. It noted that less than 1 percent of Android users on Google Play are running the 2011-era platform (0.3 percent as of late October 2018). We'd add that it might simply be too difficult to support software that far back, especially if there are planned features that might create conflicts. When there are more than 2 billion active Android users, though, the decision will result in millions of people gradually losing access to the latest apps and Google services.