It really depends on your needs and what people you communicate with are willing to use. A few platforms that are notable in no particular order.
SimpleX Chat is probably the gold standard right now. It uses absolutely no user IDs such as phone numbers, no usernames, no random strings of text. Instead, it creates unique, pairwise decentralized message queues for every single contact you have. Because there is no global identity, there is no metadata connecting your conversations together.
Session is a popular Signal alternative. It doesn't require a phone number and routes your messages through an onion-routed decentralized network that's similar to Tor. Since your IP address is hidden and messages are bounced through multiple nodes, no single server ever knows who is talking to whom, stripping away metadata.
Jami is completely decentralized, open-source platform. It uses Distributed Hash Tables to connect users directly to one another without a central server. Notably, it supports high-quality voice and video calls.