You can already do this in C though, If EVER SO SLIGHTLY wasteful in non-practical terms.
If you need a 23bit object you just structure it to be that. It’s a couple of AND or SHIFT ops when accessing, but so what? Even for 100Gbit networking you aren’t going to max out even a slightly appropriate CPU.
The same argument could be made to get rid of all numeric types except the largest. That's likely how it would compile down to on platforms without 23 bit types though just it would be handled automatically based on target. I think the point of such a feature is to abstract you from thinking about if the machine has a native 23 bit type the same way you don't think if the machine has a native 64 bit type or a hardware float type today. Also when you do this manually you're now responsible for tracking the actual type and such too. Beyond that you also want to do operations on these fields not just store them and ignore them, does the IP match this policy? Have I learned this MAC? Is this TOS in an allowed range or does it need to be bleached? Constantly pulling these out and putting them back in, the above work isn't a one time thing.
100 Gigabit networking eats more CPU than you think, especially if you're actually looking at headers. It's an enormous portion of cloud CPU usage and a big reason networking is still driven by what's easy to put in an ASIC vs running the easier thing in software.
If you need a 23bit object you just structure it to be that. It’s a couple of AND or SHIFT ops when accessing, but so what? Even for 100Gbit networking you aren’t going to max out even a slightly appropriate CPU.