Try this in your pxt.json

    "hidden": true,
    "searchOnly": true,

Thanks @ramseyball!
I've just tested it with an approved extension and it still appears on the search (and adding the extension to a project links to the latest version, which does include the changes in pxt.json). So it looks like this might need to be implemented in pxt/pxt-microbit.

@microbit-carlos why approve it if you don't want it to be in search. That is the only additional feature approved extensions have unless you specify the category. I am not sure I understand this feature request.

One possible use case could be a "dependency only" extension that could be useful to have approved, so that it can take advantage of features like the extension being cached. For example, pxt-ws2812b is a dependency to pxt-neopixel, it currently appears on the search, but when the user selects it it doesn't load any blocks, which can be confusing.

But, in this specific case, this ticket was raised for CreateAI. The ML extensions cannot do much without the CreateAI editor injecting the model and additional info into a project, so it doesn't make sense for the user to load the ML extension on a normal MakeCode project from the extension search screen.