Stumbling upon the deprecation warning of
modus-themes-no-mixed-fonts that was set to
nil in my case, I was briefly confused by the meaning of
modus-themes-mixed-fonts and its default value of
nil. I personally like when docstrings say which value does what, e.g. in the "Non-nil to ..." format.
Just a minor wording suggestion. Not sure if it fits your style.