Add back info on typelevel combinator calculus

Author: smarter

docs/docs/reference/dropped/type-projection.md

@@ -10,5 +10,8 @@ Dotty disallows this if `T` is an abstract type (class types and type aliases
 are fine). This change was made because unrestricted type projection
 is [unsound](https://github.com/lampepfl/dotty/issues/1050).
 
+This restriction rules out the [type-level encoding of a combinator
+calculus](https://michid.wordpress.com/2010/01/29/scala-type-level-encoding-of-the-ski-calculus/).
+
 To rewrite code using type projections on abstract types, consider using
 path-dependent types or implicit parameters.