If you kill one of Ediff's vital buffers, then try to leave Ediff (by
typing q in the control window), there is an error.

The error is:
ediff-visible-region: You have killed a vital Ediff buffer---you must leave
Ediff now!

Recipe from 'emacs -Q' (Windows GUI build):

* Visit a file under version control with unstaged changes.
* [M-x ediff-revision RET RET RET RET]
* In the main frame, kill one of the buffers being diffed
* In the Ediff control window frame, type [q y]

Bisected to this commit:

a26a8cc1c85f29fb11209c16d53a8ae4e4ab7ced
Author: Juri Linkov <juri@linkov.net>
Date: Sun Nov 10 00:04:13 2019 +0200

'y-or-n-p' now uses the minibuffer to read 'y' or 'n' answer (bug#38076)