When it was still called Facebook Messenger, it added end-to-end encrypted chatting. Many things have changed, but the optional feature is fully rolled out to everyone. Some regulators claim that changing to E2EE as a default would harm public safety, so that may not happen until next year.
Messenger users can either use vanish mode to enter one of the secure chats or use the original version that was introduced in 2016 as Secret Conversations. When you start a new chat, you can turn that on byggling the lock icon.
Messenger has some new features that can be enabled as well. In end-to-end chats, you can use stickers, reactions, and long-press to reply or forward messages. The chats now have verified badges so that people can identify authentic accounts.
You can save media in the chats, and there will be aSnapchat-style notification that will be rolling out over the next few weeks.