Some modern headset designs don't use the top cap to do that. There's a bunch of products that exploit that to put toolkits, spare CO2 canisters or whathave you in that spot.
replySure, 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?
replyPS. 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.