but if you have a single bool in a stack frame it’s probably going to be more than a byte.

Nope. - if you can’t read RISC-V assembly, look at these lines

        sb      a5,-17(s0)
...
        sb      a5,-18(s0)
...
        sb      a5,-19(s0)
...

That is it storing the bools in single bytes. Also I only used RISC-V because I’m way more familiar with it than x86, but it will do the same thing.

on the heap definitely more than a byte

Nope, you can happily malloc(1) and store a bool in it, or malloc(4) and store 4 bools in it. A bool is 1 byte. Consider this a TIL moment.

things that store it as word size for alignment purposes

Nope. bools only need to be naturally aligned, so 1 byte.

If you do

struct SomeBools {
  bool a;
  bool b;
  bool c;
  bool d;
};

its 4 bytes.

I don’t think so. Apart from dynamically typed languages which need to store the type with the value, it’s always 1 byte, and that doesn’t depend on architecture (excluding ancient or exotic architectures) or optimisation flags.

Which language/architecture/flags would not store a bool in 1 byte?