Re: Eliminating "changed in Emacs outside of Customize"

[Richard Stallman]

It is normal and not unusual for Lisp code to set buffer-local values
for defcustom variables, and to bind them with `let'.
Consider case-fold-search, for instance, and paragraph-separate.

It is not clean for Lisp code to set case-fold-search globally, since
it is cleaner to make a let binding instead.  But I would not endorse
the idea of making a "rule" against this.