it was deprecated in 2.12.

recently it was copied into the scala-parser-combinators library, which was (hopefully) the only thing using it: scala/scala-parser-combinators#97

but there isn't yet a scala-parser-combinators release that has it, so we can't actually remove it from the standard library quite yet. well, I suppose we could speculatively remove it on the assumption that either a scala-parser-combinators will be published in time, or we'll unbundle in time. but anyway there's no rush, just something to get back to, hopefully for 2.13.0-M1.