Just checked with a 2015 model, a magnet won't stick to the stays so they're almost certainly stainless.True! The steel ones vary a lot in their susceptibility to rust, though. I think some may be stainless, and some galvanised. The ones in my spares box certainly look and feel like stainless. In contrast, the ones on a 2005 bike I recently bought were ruined.
I have aftermarket Ally mudguards on one of my Brommies, they came with Ti stays. The standard stays are pretty minimal in weight anyway.