This is not going to be backwards compatible and it may surprise people by "properly" closing polygons that previously would have failed because they were not closed _without_ any mechanism to throw a validation error of some sort as suggested by @chilling in #4085.

Personally, I think that I prefer the concept of autoclosing being non-default behavior because it matches existing documentation. However, maybe in a major release (2.0 perhaps or even a 1.x where x is a few releases later?) this could be changed to default to autoclose because it prevents the need to waste sending the extra point.