- Is @param.context("ctx") a valid decorator in Indie v4**+**?
No, there is not
- If ctx doesn't need to be declared, how do I pass it correctly?
In Indie >= v4 you do not have to pass it directly. Instead you should use self.ctx, where self - is a reference to Main class (or any other Context).
- Is there another way to define context-based parameters in Indie?
No, and... you should not need this. A context is an abstraction of a trading instrument (a dataset of candles in other words). If you need additional instruments (i.e. Contexts) you should use Context.calc_on method.