Would like to know why I’m banned, and for how long
You need to contact the developers at forzafb@microsoft.com with full information of all gamertags used on your console, the message you’ve received in-game and any other pertinent information. They administer the Xbox Live Code of Conduct in Forza Horizon, issuing bans.
Neither bans nor the enforcement of the regulations are discussed on these forums. You’ll receive an automated response with links to various information, and that is your assurance your message has reached the proper location.