@SGIII, I missed one step. These are the correct steps, I think.
To attach the handler to the current Outline, we should call onCellChanged method with the handler as the parameter and store it in a variable.
I would type (in the Console):
const redHandler = PlugIn.find("com.unlocked2412.conditionalFormatting").handler('redHighlighting')
Attaching the handler and returning a PlugIn.Handler.Registration …
const registration = onCellChanged(redHandler)
To detach the handler, I would type (again, in the console):
registration.remove()
