It's a debugging feature that can go away after the whole goal is finished.
In order to be able to debug and test the new infrastructure for jobs, support gradual job-by-job rollout and test the new infrastructure without disrupting old, proven to work, job queue implementation, we need to post events both to the old job queue and to the #eventbus .
The easiest way to achieve that is to introduce a new hook `AfterJobSubmit`, listen to it in the #eventbus extension and try posting the `IJobSpecification` to #eventbus. Then we'd be able to experiment with new JobQueue without disrupting what's working now, and, once established the queue works for a particular job, switch the queues one-by-one.
Since the hook will only be useful during the transition period only within WMF infrastructure, it should only be enabled in the particular queue we are using, `JobQueueRedis` and it would be clearly documented as `deprecated` from the very beginning.