It’s true that this may result in run-time errors, but only if someone uses the obsolete DU case in the code, so in a way there’s already a warning somewhere that’s being ignored. Perhaps a better option would be to add additional text to the obsolete warning like:
“This construct is deprecated. My own additional message.” → “Using obsolete union cases may result in incomplete pattern match exceptions. My own additional message.”
I don’t really understand why if it’s deprecated and you don’t want to match on it anymore, why you wouldn’t just delete it.
Older parts of the code may still use it.