Huisheng Xu: I was not aware of this issue and its bugfix. Thank you for pointing out. Nevertheless I don't see why adding this method to ExternalActivityBehaviour is not a good design. Ronald has been thinking this carefully. When implementating custom nodes which also need a timeout behaviour this would be already useful.
Please also consider Maciej's point:
For me cancelling a task and timeouts have to be clearly distinguished from the completion and deletion of tasks because they all mean different history data being related. If there is an escalation/timeout I don't want the task to be treated as if it was deleted. Deleting tasks should only be possible for tasks not belonging to a process/execution.