Moved five methods from LinearSeqOptimized to List
List ain't the only LinearSeqOptimized in town: that commit changed the performance and evaluation characteristics of the other subclasses: Stream, Queue, Stack, and MutableList.
scala> Stream.continually(()).filterNot(_ => false).take(2)
I've only highlighted the first example I've found. I really don't see how that change can be allowed – even if you were to copy/paste the removed methods into those other subclasses, you would have broken any third party code extending this trait. You could take the position that it's just an implementation trait, which would let you remove if from the parents of List.