I just had one of those “Well Duh! Of course” moments. It came to me with this Emacs Command of the Day tweet:
M-x read-only-mode, bound to C-x C-q.
Avoid accidentally modifying a buffer with this handy minor mode!
— Ⓔ Command of the Day (@ecotd) November 28, 2017
I always think of Ctrl+x Ctrl+q as turning off read-only-mode in a grep or Dired buffer. See Mike Zamansky’s video on Dired for an example of the latter. But, of course, what it really does is toggle read-only-mode so if you want to put a buffer in view-mode to prevent inadvertent changes you can just use Ctrl+x Ctrl+q to turn it on.