Platform filters are tags. You just can't select them in the tag box as a user nor developer.
There is also https://itch.io/games/released which you will not find at the filter options if none are selected. At least you find that one under "Directory". But you will not find https://itch.io/games/made-with-unreal-engine there.
The way this is currently done when publishing is per file. You designate a downloadable file to be for one of 4 systems. I suspect getting the ios tag is dependent on having a link to the apple store. And getting the web game info is implicit by having a web game.
There could be another metainformation tab for projects that have files usable for retro consoles and the like. You could select those like you could select an engine.
But having those meta tags still does not give you a selector in the filter list on the left side of the page. And there are currently about 332 platforms. That filter list selectors are for the most used options and consoles are not most used search options.
Regular tags are the best choice at the moment. What exactly would be wrong with using tags? That is what I do not get. Yeah, there are technically like 300 more platforms than listed there, but are those 300 platforms used to actually browse games here? Can you browse Itch with a NES and download files that are native to your system? No. Having a prominent filter for webgames and the major platforms that allow installing files makes a lot of sense.