The ease of dealing with arbitrary bit-width integers and packed structs is actually one of the 'killer features' for me in zig.
Zig natively supports arbitrary bit-width integers, the ABI is defined and you could simply think it as a slice of the next larger backing integer.
The[3]u8 to u24 bitCast will simply be backed by a 32bit int, using the same ABI. As you have u1 - u65535, sometimes it can be multiple words.
The 24 Bits (3 Bytes) [3]u8 to u24 example is exactly related to utf-8 that covers all the languages but excludes the emojis.
There are very valid use cases when you want to limit utf-8 to U+0000-U+FFFF, and it is valuable if your language allows you to make those decisions.
Remember, in zig packed structs are just integers and integers are just a group of logically consecutive bits.
Arrays like []u24 do not have the same ABI, arrays are not bit/byte packed, are not universally LSB across archs etc..
The compiler isn't producing unaligned code, don't confuse the abstraction with the concrete implementation. And yes [8]u1 and [8]u8 are exactly the same size and shape, even though they are arrays.
My current project is parsing ELF/Macho files, I can easily have zero allocations in my hot path with zig, the same is far more challenging in C, so I am biased, especially with zig allowing methods on structs.
And yes, I do use that crazy casting to 0xdeadbeef and other ascii metadata that is in those files.
To be clear here, I am not trying to prove you wrong, this is one of the places zig is very different and (IMHO) useful. Especially with streaming data or where you have network ordering etc... It is so nice to only cast what you need to but it does take a little while to wrap your head around how this interacts with buffers which are not your native endianness. At least for me, once I figured out to separate the shape of those data streams from their values it was super useful.