Since I updated the Modus themes (9521dcec) and my configuration (modus-themes-slanted-constructs to modus-themes-italic-constructs), I get the warning:
Warning (defvaralias): Overwriting value of ‘modus-themes-slanted-constructs’ by aliasing to ‘modus-themes-italic-constructs’The documentation does not provide an answer on what to do with the old definition but it makes sense one can only define a variable once (the old definition is a proxy for the new one).