> I'm hesitant to make it line-buffered by default when directed to a 
> file, since this could significantly slow down a program that for some
> reason produces super-voluminous output (e.g. when running a program
> with heavy debug logging turned on).

Is that really the purpose of standard error though? Heavy debug output, in my experience, is usually sent to standard output or to another file.

Also, did anyone ever complain about this as a problem, given it is the default behaviour of Python 2?

In my view the requirements of seeing errors when they happen, and guaranteeing that they will always be seen no matter what happens afterwards, should weigh more heavily than this.