According to a post on the Signal blog, a federal grand jury in the Central District of California has subpoenad Signal for shedloads of user data, like subscriber information, financial information, transaction histories, communications, and more.

However, Signal simply doesn't have the data to provide because it does not have access  messages,  chat list, groups, contacts, stickers, or profile name or avatar.

The only things that Signal can offer up to the court are Unix timestamps for when the accounts in question were created and last accessed.