It's also not rare to find wrapper scripts that check "was there any output to stderr?", and if so, consider the program to have failed. Warnings are not errors

It's also not rare to find wrapper scripts that check "was there any output to stderr?", and if so, consider the program to have failed. Warnings are not errors, but also not output, so there isn't a good way to report them in the stdout/stderr model (but stderr is still better than stdout).