That wasn’t my story — but it is an interesting question. Most financial firms require you to submit to mandatory arbitration of disputes, as part of your account opening contract.
Here’s why: https://www.bankrate.com/banking/beat-the-bank-in-mandatory-arbitration
You’re probably referring to a December 11th deadline that Chase has given to opt out of arbitration. Here’s the link to the form letter that must be submitted carefully to the noted address.