Boarding has been fully migrated to the new app. 
If you own racehorses (the only horses that can be boarded), you will see “Boarding” in the menu.
Clicking on that will show you a page with your current boardings, as well as history (for current racehorses only).
Current Boardings shows you who is boarded, for how long, and gives you the option to stop boarding.
Boarding History shows you who was boarded, where, and for how long.
If you have horses that can be boarded (e.g. they are racehorses, they’re not in transit, they’re not at your farm, they’re not currently boarded), the Create Boarding button will show a form to select a horse to board.
You can also start/stop boarding from an individual horse’s page.