When a player does not send the profile public key on login in ServerboundHelloPacket, messages of that player will not have any special mark to tell other players that the message cannot be validated (no matter if it is a valid signature or not). These messages will also still be shown when you have the "Only Show Secure Chat" option enabled.
Nevermind, local lan players always have everything show as secure, which looks like it's intended.
Actually nvm, you can kill this, apparently local hosts have special handling when it comes to verifying messages.