I have no problem with pinging Python-Dev, or any other forum, asking 
for interested parties. I just don't think we should be intentionally 
spliting the discussion more than it's going to get split organically. 
(If this is interesting to people, they will discuss it on other forums 
no matter what we say or do.)

Aside from the conservative position "If it ain't broke, don't break 
it", also known as "Chesterton's Fence":

https://matt-rickard.com/chestertons-fence/

I'm not specifically opposed to this change. Nor am I in favour.

Why do we print the first line of the doctests in the output of failing 
tests? If we understood the rationale for why we do that, it might give 
some insight into the consequences of removing it. If we don't know why 
it was shown in the first place, maybe we shouldn't remove it until we 
have some idea.

I am -1 on another option flag unless there is no other choice. Each 
boolean flag option doubles the number of tests of unitttest itself 
needed for full coverage.

Another option might be to leave the docstring line alone, and just 
*add* the 'ERROR' to the method name line:

    # current output
    test_broken_doc (mathlib.testing.test_utils.TestMinmax)
    This is a docstring ... ERROR

    # enhancement
    test_broken_doc (mathlib.testing.test_utils.TestMinmax) ERROR
    This is a docstring ... ERROR

That seems to me to be less risky than trying to cram them all on one line.

    test_broken_doc (mathlib.testing.test_utils.TestMinmax) This is a docstring ... ERROR

By the way, I presume we do the same thing for FAIL lines, which have 
the same behaviour as ERROR lines:

    test_fail_doc (mathlib.testing.test_utils.TestMinmax)
    This is a docstring ... FAIL

    test_fail_nodoc (mathlib.testing.test_utils.TestMinmax) ... FAIL