Local actions were using YAML derivatives on top of annotation-based discovery. This change makes the YAML the primary discovery mechanism. Since the YAML file format is unchanged, existing local actions still work, but the actual plugin ID may change since they are no longer derivatives.
If you need to override the LocalAction class, specify the class key in the YAML file.
In addition, the LocalAction plugin class has been improved so that it handles routes with parameters, and tries to supply route variables based on the current request automatically.
Also updating the prior change notice: https://drupal.org/node/2007444