We cannot make every variable interactive because it would kill the completion interface and allow random users to modify internal settings in unexpected and potentially inconsistent and disastrous ways.

But I would risk saying that there are not that many global variables with the sole purpose of modifying the way interactive commands work and that are safe to be modified and sufficiently documented.

26 sty 2024 17:19 Eli Zaretskii <eliz@gnu.org> napisał(a):

We cannot possibly make every variable a defcustom, just because
someone somewhere will need to set a variable while not being a
programmer, can we?

So, while I understand your motivation, it still doesn't convince me
to make the variable a user option. Splitting stdout and stderr might
be a basic usability requirement in your eyes, but IME users rarely
send stderr to a separate place, which is why this variable is not a
defcustom.