[TUHS] Why does shell write to stderr?

[Richard Salz]

Looking at
https://unix.stackexchange.com/questions/380012/why-does-bash-interactive-shell-by-default-write-its-prompt-and-echoes-its-inter,
in particular the first answer which has a link to
http://minnie.tuhs.org/cgi-bin/utree.pl?file=V7/usr/src/cmd/sh/main%E2%80%8C%E2%80%8B.c
Made me wonder: why?
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