IANAL but it depends. In the US there is strong protection for the contents of your mind and self-incrimination. So if your keys were locked behind a strong password the legal system wouldn't be able to access it. But if you had no password they would be able to seize the device and read the messages.

So basically if the messages are inaccessible other than a secret that you know them yes, they wouldn't be forced to reveal it.

(Also not a Lawyer) I’m not familiar with the laws in Nebraska, but they wouldn’t be able to get the messages from Meta. They would need to get on the devices, but it seemed like the people charged themselves tipped off about using Messenger to the police. The only other way to get E2EE message from a device without consent is with the use of force.

From the article:

However, campaigners note that Meta always has to comply with legal requests for data, and that the company can only change this if it stops collecting that data in the first place. In the case of Celeste and Jessica Burgess, this would have meant making end-to-end encryption (E2EE) the default in Facebook Messenger. This would have meant that police would have had to gain access to the pair’s phones directly to read their chats. (E2EE is available in Messenger but has to be toggled on manually. It’s on by default in WhatsApp.)

(…)

However, private chat messages are only one component in a whole range of digital evidence that is likely to be used by police to prosecute illegal abortions in the United States. Investigators will be able to request access to many data sources, including digital health records, Google search history, text messages, and phone location data.