I see. So, to sum up, the general guidance looks like this:
- It's recommended to use detrending in Train almost always, unless the strategy exploits long-term trend.
- It's not recommended to use detrending during Test, unless the strategy is very asymmetrical, i.e. long-only.

Does it make sense?

I also have a question:

Is DETREND intended for observing DETREND training's effects on testing, or is it supposed to be used for code in production as well?

That is, would there ever be a reason to train data with DETREND and use that code in production?

Yes. You normally want to trade with parameters that work independent of trend.