Looks like this allows something that was supposed to give a warning (according to #12706 (comment)):

@deprecated("no CaseClass") case class CaseClass(rgb: Int)

object CaseClass:
  def answer: Int = { val x = new CaseClass(42); 42 }

val a = CaseClass.answer

I disagree with the assertion that the usage in the companion should warn. Deprecation is for API (you can't deprecate local methods, for instance). The interface between companions (in this view) is not deprecatable API. I might deprecate the constructor in favor of the companion apply, for instance. I don't intend to deprecate the method.

(The bug in old Scala 2 was that it intended to deprecate the companion, but did not.)

I'll put more thought into the use cases and whether the community build had an opinion on Scala 2.

The other example from the linked ticket is scala.io.Position, where object Position extends Position and the class is deprecated.

Clients should not use the class but may use utility methods in the companion.

I think the synthetic methods should trigger the deprecation warning as well; the following code should definitely trigger a warning (and with this change it does not):

@deprecated("no CaseClass") case class CaseClass(rgb: Int):
  def magic(): Unit = ()
  
val a = CaseClass(42).magic()

Closing until Scala 2 ticket is sorted.

Note to future self: I changed my mind since Valentine's Day.

The ongoing adjustment on Scala 2 is at scala/scala#10311