@jamie The 8-bit guy made exactly the same joke a few years ago: for some reason, he couldn't use the Ctrl key as a control in his experiment comparing various #retrobrighting methods.
And I also couldn't, because both my Ctrl keys are visibly less yellowed than adjacent keys.
I've seen keyboards whose space bar is more yellowed than the rest, but that's the only key having no engraving so it's plausible that it went through a different process.
So, why Ctrl?? 🤷