Ævar Arnfjörð Bjarmason [off-list ref] writes:

quoted

Here's a past discussion (that actually goes the other way: somebody
complaining that stderr should be on stdout!) where I laid out my mental
model:

  https://lore.kernel.org/git/20110907215716.GJ13364@sigill.intra.peff.net/ (local)

...and a third way (which git doesn't conform to at all), which is that
std*err* is really what we should be using for errors only.

You shouldn't write anything that isn't an error there, or at least
that's what I've seen some software in the wild assume.

That is already covered in the old thread Peff cited (not in the
message from Peff, but you can find what others said on the topic
back then from that page).  We saw programs that declare any output
to standard error by programs they spawn indicates an error, instead
of checking the exit status of the programs they spawn, which would
of course break when they try to drive git.