I'm not talking about a hex key that extends in length but one where it can collapse in on itself to increase in size.
I did some digging online but all I could find was a patent from a company called TeleHex but it kind of sounds like they came and went in the mid 2000s. They're website doesn't seem to exist.
I feel like it could save space in my pocket compared to a typical hex key set or a multibit.
I was hoping I could at least find a bootleg version on Ali Express.
True. You'd be giving up a bit of convenience for a bit of compactness. It wouldn't be too bad with three separate tubes for Allen, Torx, and Phillips/flat/square