Enabling encryption by default will mean that Meta should not only be unable to see the contents of most Messenger chats but that it also won t be able to hand them over to law enforcement, , messenger, End-to-end encryption, engineers, cryptographers, designers, policy experts, META
Messenger previously had the option to turn on end-to-end encryption, allowing a message to be read only by the sender and its recipients, but with this change messages would be encrypted by default, Meta said.
According to The Verge, while this is a good step, end-to-end encryption for group Messenger chats is still opt-in for now. Instagram messages are also not encrypted by default, though Meta said in August that would happen "shortly after" the rollout of default private Messenger chats.