nothing happens. MS is just stopping updates, bug fixes and support. the Xamarin tools may disappear in future versions of visual studio.
Your code will still run. The most typical consequence is that the mobile O/S may make a change that breaks your code or the Xamarin runtime, so your code will not run on that version of the O/S without a fix. I assume the repo will remain open source, so you can clone and fix the runtime yourself, but this may not be practical.
with IOS, code signing may be the first issue you run into. You are often required to update Xcode to sign, and typically you need a Xamarin update for each new release of Xcode.