Sorry for my super late forum responses lately. Yes it kinda causes problems to just use the oldest platform for a few things. Yes, it's good for some platforms especially the ONE/XSX co-branding (it is a fantastic solution) but it causes issue for the Sneak King example since it has 360 box style but now it's been changed to OG xbox, and the same would apply for the demo discs which co-branded versions look more like 360 discs but now they would become just og xbox. It would also cause issue for stuff like the Red Dead Redemption 360/ONE version since it has ONE box style but now would become 360, further complicating the 360 category as there are already plenty of different 360 versions of RDR there. Lots of people who would see the box at first glance wouldn't realise it's not JUST a ONE game (although it's clearly written Xbox 360 / Xbox One with big white letters) and I can personally confirm this as, it is time for me to make a big reveal, I manage the electronics department at the largest Walmart store of central Québec. I can ATTEST that most people don't even take the time to read and just assume it's only a Xbox One game just by taking a look at the box style. Even worse, the manufacturers who send me the stuff, as well as all of our own internal systems, always refer to those games as XO (walmart's abbreviation of Xbox one) not only on all the 360/ONE co-branded items but also on the ONE/XSX titles, while 360 titles used to have their own category obviously and XSX only games are indeed referred as XSX in our internal systems.
TLDR listing as older console will not always give the best results. Could be used for the ONE/XSX titles but for the rest it should just be something like "Co-branded titles should be listed under the console which the box size/style most closely resembles". But that's just my personal opinion