From what I understand about vanilla chat behaviour (the red bars/(!)-things that were added in pre2) and MC-253517, online-mode=false servers should never be treated as "dangerous", even if their enforce-secure-profile=true, because the chat is still considered unsigned despite what is asked from the client.

Correct me if I'm wrong.

This should be fixed by 0f05789, since the mod now has some capacity to dynamically detect server demands while logging in. Servers in offline mode will not require you to expose your public key even if enforce-secure-profile is enabled, and will not ever relay your key to other players.

However, if you log into offline-mode server without this mod (or some other mod that strips signatures) installed - your client will still send signed messages, and server will still relay them complete with signatures. Other clients will not be able to verify those signatures since server does not give them your public key; however, if those signed messages get reported to Mojang - they will be able to verify them.