Alternatively "disimprovement".

Via https://dict.leo.org/forum/viewUnsolvedquery.php?idThread=49... and https://dict.leo.org/forum/viewUnsolvedquery.php?idThread=16...

As a German, I might not have thought of that but would absolutely use it to translate Verschlimmbesserung. Does it sound "normal" enough to native English speakers?

I prefer “misprovement”, as in “misfeature”.

Isn't the English word more or less "regression"? If you want to coin a new word, how about updowngrade?