You may actually be right; I know that SSH channel protocol has special message kind specifically for sending signals to the remote process, which is different from channel messages with normal data, but I don't know if it's used by actual ssh client implementations. They may simply just put local tty in raw mode and forward all input from it as normal data.

Hard to say without looking at the actual code, and right now I am not particularly in a mood for reading C sources at the moment. Maybe someone else is and will tell us the true story!

P.S. Now that I think of it, ssh implementations have to "sync" local and remote tty parameters or at least make it look sane for the user: if you resize your local xterm, arguably the remote e.g. vi should get notified, but what if it's the remote process that changes the terminal dimensions, should your local xterm get resized as a result?