Oops I was still typing and finding links before posting! You've found it now
There will be no more user based boarding, all user owned boarding has closed and boarding horses away is now handled by a game run boarding center. A news announcement was made on this today https://horsefable.com/topic/8487/ and prior discussion on the upcoming change had been going on since May as seen in the premium corner thread titled โ[INPUT] Boarding Systemโ (not sure I can link the thread as it is a premium corner thread).
You didn't do anything wrong. There were changes to other premium features like training centers, you'll want to check that news post for details and frequently check any new news announcements if you want to know upcoming changes.