I still think it would benefit to have an "other" choice for devs to explicitly say "this isn't meant to run natively on the main platforms".
This does not work the way you think it works.
Platform meta tags are not mutually exclusive. It is only the filter that makes it look like that, since you can only chose one of the options on the website. But you can have a game that is checked on all or multiple platforms, and that is rather the normal case. Most engines can export to multiple targets, so the projects typically have excecuteables for several platforms and often for web too.
You can create a filter by hand, to see this.
This is about 30000 games. https://itch.io/games/platform-linux/platform-web/platform-windows
If you search for a dreamcast game, you go for the dreamcast tag. Directly. Hence my statement above that one would not select "other" first and select "dreamcast" next. It makes no sense at all.
The only downside is, that some games will be in that list that might not have a natively runable rom file or only be "inspired" or whatever. So what. Tags on Itch are inherently inaccurate, no matter which tags. The developer choses them. Not a curator, not a majority popular vote. You will even find games here that check the box for android, even though the game only runs with a helper app. It is tags, not software specs. This goes for the platform tags as well. It does not even specify which windows or which android version. Will a game with platform android run on your old android tablet? You do not know by that info alone.
From what I can see, we don't seem to have any tag for SNES ROMs.
You mean that one? https://itch.io/games/tag-snes and this https://itch.io/games/tag-snes-rom ?
Itch has free tagging. There are just too many tags to put them all in the suggested tags box. You find the snes roms under snes. Including the three games that used the more complicated snes-rom tag.
why I think console-based platforms should be included
I think only the major systems that are actually used to browse on Itch should be included in that special treatment. Plus web games. You do not browse Itch on a dreamcast, nor on a snes. You might browse Itch on a current console, but you cannot easily sideload games on current consoles.