It has already been said but to make it officially: The project targets a release per month. Deprecated code is removed in ten releases.
It has been pointed out in Three.js, semver, and addons - #7 by donmccurdy that separating addons from core is not required for having semver.
The OP asked a quite simple question and there is no need to start a discussion about semver and addons here. We already have a dedicated topic for this: Three.js, semver, and addons