Comment by pandaman

Sure, threaded headsets (hardly modern, but they do still make bikes with those) don't use top cap, but they also don't have a hole at the top, which products you are talking about?

PS. I searched and found some in-steerer storage. It's not a headset but essentially a hollow compression plug replacement with a top cap moved to the bottom. I have never seen a road fork with a hole at the bottom so it is likely limited to MTB forks and, since there is no compression in that thing, it might be not very safe with a carbon steerer.