Some of them are probably exposed to smoke but from what I understand a lot of this yellowing is the fire retardant being drawn out of this type of plastic due to age. I've seen some of this yellowed plastic become more brittle and powdery too (early FC consoles).
Instead of peroxide you could just strip the console down and give the plastics a good soak in mild soapy water overnight. Then dry thoroughly and spend some time going over the yellowing with magic eraser repeatedly. Won't get rid of the cosmetic damage completely but should at least lighten it. So in milder cases of yellowing the console should look "cleaner".
The problem with the old FC consoles is that they have stickers on top so you need to source replacements if you want to attempt a good clean.
For SFC, honestly imo if you want a clean unit then find a clean shell or a couple of shells that you can combine parts to make a nice one. I mean you might be able to source a cleaner shell from a dead aka junk SFC and use it for the yellowed SFC you already have that is working/refurbished/modded.
If money isn't a barrier then import a CiB SFC with no visible sign of yellowing
I have two SFC's, both have some yellowing but one is much better than the other. One I bought as "junk" with an early FC (with AV & USB power mod) plus a small box of carts. The other I found at work, nothing with it, so I got it as junk as it couldn't be tested for sale in the shop.
BTW that early FC I have that is modded doesn't have noticeable yellowing.
