I think it might be a bit more than that. As far as I can see, all the primitive data types in Rust have sizes which are multiples of eight. However, there are still architectures out there with 9 bit bytes, or 36-bit words. Maybe it's good that all the primitive data types have an explicit size (encoded in their name), but this comes at a cost (in case a byte is 9 bits long, one of the bits will effectively not be used). On the other hand, C and C++ are less explicit about the sizes of their data types so they can accommodate these more weird architectures more easily (and, if you need an explicit size, you can explicitly ask for it).
You have a point but in general you wouldn't write "generic" C code on these exotic (by modern standards) architectures. So I guess if you wanted to port Rust to these systems you could introduce an i9 and an i36 and use that instead. I doubt you could get firefox and its dependencies to work on a CHAR_BIT == 9 system without some heavy modifications. I could be wrong though, it's not like I tried.
In my experience most modern codebases tend to target POSIX systems (explicitly or implicitly). That gives you a subset of C to work with and makes your life easier. The exceptions are things like DSPs which can have rather non-standard layouts but you generally write very specific and unportable code for these anyway.
Of course if you want to be completely portable you can't assume that CHAR_BIT == 8 or that you can convert back and forth between function and data pointers but in practice a lot of software relies on this.
Even before stdint became part of the standard it was very common for programs and libraries to typedef their own "sized" integer types. It's the only reasonable way to deal with ints in C IMO, otherwise you can't assume that your ints are greater than 16bits (which is probably too tiny for many uses) so you're tempted to put "longs" everywhere. But then longs will be 64bits on modern architectures which could be overkill and reduce performance.
So in theory those variable-size integer types are interesting but in practice they're basically useless because you end up making assumptions, one way or an other. I think it was a good idea for Rust to get rid of them.
