[ruby-core:109353] [Ruby master Feature#18930] Officially deprecate class variables

Issue #18930 has been updated by Eregon (Benoit Daloze).


For such a case it seems fine to use instance variables on the class and expose e.g. `SomeThingNeverInherited#some_config?`.
If you don't want that to be public API it could be `:nodoc:` or made private + using `send`.

```ruby
class SomeThingNeverInherited
  @some_config = :initial_value

  class << self
    def some_config=(value)
      @some_config = value
    end
    
    def some_config?
      @some_config
    end
  end

  def do_thing
    if SomeThingNeverInherited.some_config?
      do_one_thing
    else
      do_another_thing
    end
  end
end

SomeThingNeverInherited.new.do_thing
```

So I think it is fully actionable, even though it might require some thinking especially if inheritance is used, but then the actual semantics were very likely not the intended ones, so such thinking will actually fix bugs.

----------------------------------------
Feature #18930: Officially deprecate class variables
https://bugs.ruby-lang.org/issues/18930#change-98497

* Author: Eregon (Benoit Daloze)
* Status: Open
* Priority: Normal
----------------------------------------
Ruby's class variables are very confusing, and it seem many people agree they should be avoided (#18927).

How about we deprecate them officially?

Concretely:
* Mention in the documentation that class variables are deprecated and should be avoided/should not be used.
* Add a parse-time deprecation warning, now that we only see those with `Warning[:deprecation] = true` it seems reasonable to add.



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