> I would guess that if the FBI could merely have a list of who their suspect contacted over an app, and when

Well with WhatsApp they most definitely can, but it has never been a secret. WhatsApp always had access to the metadata (whereas Signal makes a lot of effort to reduce the metadata they have access to). In ~2016 WhatsApp integrated the Signal protocol to add end-to-end encryption, but did nothing about the metadata.

Again: if you care about privacy, use Signal.