I believe that all your complaints would be solved if you set the env var
BAT_THEME=base16
right? (Or indeed ansi-light or ansi-dark; none of those themes use hard-coded colors).> Asking for accessibility in productivity tools is not mounting "a high horse"
Can you see my confusion though. Look again at your language (and your italics):
> `bat` is one of the examples of when 256 and true colour shouldn't be used.
You appear to be not asking for accessibility but saying that bat shouldn't support color themes that make use of 256 colors. And that is where you are transitioning from perfectly reasonable to perfectly unreasonable :) It's not up to you how other people use their terminal. If they like using more than 16 colors then that is perfectly alright and none of your business.
As I mentioned above, that transition from asking for (even "demanding" would be fine) accessibility to cutting off other people's noses to spite their faces is where accessibility concerns have unfortunately led to terrible vandalism: the removal of thousands of hours of video lectures from the internet by UC Berkeley.