This "unwritten rule" stuff feels like a weird holdover from another time. Seeing Duffey throw at(?) Mercedes, I feel like I'm watching an old movie where a character walks into a bar and gets kicked out because the regulars don't like the look of him. It feels anachronistic. It feels like it comes out of some sort of macho code that hasn't existed since before I was born.
Speaking as a guy who's only been following baseball for a couple of years, I'm not a fan of this. This is much more of a turnoff to me than long late innings or frequent pitcher changes, or whatever else MLB has been trying to tweak to make baseball friendlier to newcomers.