Deprecating right projections makes sense - Either is now right-biased, there's no need for them.

Deprecating left projections also seems desirable, but I feel this might have been done a bit quickly, with some unpleasant consequences.

There are existing scenarios where one might want to call .left.map(f) or .left.flatMap(g) - flatMap is particularly useful to recover on errors, for example.

The current state of scala means that we'll get a warning for this (or, for the -Xfatal-warnings crowd, an error) without a clear workaround. Suggestions on scala/contributors were:

e.swap.flatMap(x => f(x).swap).swap

e match {
  case Left(l) => f(l)
  case r@Right(_) => r
}

Both of these work fine, but I doubt anyone who has to replace e.left.flatMap(f) by one of them will feel it's a step forward.

Truth be told, I'd rather see projections disappear and Either get leftMap and leftFlatMap methods, but it's a bit late for that, with the 2.13.0 API being frozen.