In many cases optional sidechain inputs are not used.
Previously sidechain ports were created, but remained
unconnected and silence was passed to the plugin's key input.
Plugins can detected if a pin is connected. Some plugins
(e.g. VST3 Waves SSL Comp) activate the sidechain processing
automatically when depending in connection.
It is more common that a user does not want to use an external
sidechain, and if they want they should use the pin-dialog
to connect it. So leaving it off by default is sensible.
see also #9223
3d62ab1c53