@d-healey It's just a simple call to JUCE's repaint() method that causes the UI element to repaint. The difference to the ScriptPanel.repaint() function is that this one will cause the scripted paint routine to be evaluated again.
You're most likely need it if you're using custom LAF that use external data to paint some components so when you change that data you can call the sendRepaintMessage() function - otherwise you would have to do some hacks like changing the width by a pixel or something to trigger a repainting.
@mehmethand Just so you know : I'm on Intel Mac / Monterey / Xcode 13.2.1,
compiling plugins works fine but the message is still there. Don't mind it, it's just a warning, not an fatal error.
I've updated my coarse/fine tuning script to use the new Engine.setGlobalPitchFactor() API function. You can drop this script into any project and connect your UI knobs to it using the property editor, so you get the benefit of this functionality without having to do any scripting.