If you implement custom actions in Herald, you should now subclass HeraldAction instead of HeraldCustomAction.

The HeraldCustomAction class no longer exists, and code extending it will not load, work, or have any effect.

The API for HeraldAction differs from the API for HeraldCustomAction. You can look at the native actions (which now also extend HeraldAction) for guidance.

See T8726 for some context.