Waist packs are pretty common in the MTB crowd. They hold a bottle, sometimes two. The advantage is that you can carry a full repair kit and get an occasional sip without carrying a camelback. It works out great for sips after a climb, usually not while moving. It’s not something I’d want for an all day ride but are great for a quick lap or two.
I have one and mostly like it. Off road, as described.
I would rather light my hair on fire than use it for a road ride.