Deprecate case-variables

[Walter Bright]

https://github.com/dlang/dmd/pull/14829

Case variables are:

"The case expressions in ArgumentList are a comma separated list of expressions. 
Each expression must evaluate to a compile-time value or array, or a **runtime 
initialized const or immutable variable of integral type**. Each expression must 
be implicitly convertible to the type of the switch Expression.

Compile-time case values must all be distinct. Const or immutable runtime 
variables must all have different names. If two case expressions share a value, 
the first case statement with that value gets control."

https://dlang.org/spec/statement.html#switch-statement

Should we deprecate the case variables?

Pro: for the sake of simplicity, as it is a surprising and presumably 
little-used feature.

Con: people don't like breaking existing code, and the feature isn't causing any 
known problems.

I hope we can reach consensus.