[Task Manager] Task Type deprecation isn't possible

[gmmorris]

context

We don't have a good mechanism for deprecating Task Types.

When a Task Type is removed in a version, it is possible to end up with old tasks that are left over from older versions and these never get cleaned up.

A recent change meant that this no longer breaks Task manager (and it proactively marks them as unrecognizable), but we don't have a good mechanism for proactively handling those tasks.

This is especially pertinent with the upcoming change where we're moving work that used to be in tasks, out of them. When a user upgrades a Kibana env that has several queued up tasks, we would expect those tasks to still be executed after the upgrade, even if we don't intent on supporting those going forward.
This is even more pertinent with potential introduction of "temporary" Task Types (such as the one proposed here), which we only need for a couple of minors to address a problem, but might not be relevant after wards.

proposal

This isn't the only way to address this, so feel free to take another approach, but what I have in mind is something like this:

1. A developer can mark a Task Type as deprecated, but they still provide the Task Type implementation.
2. When Kibana finds a deprecated type - it will execute it as it did before.
3. When code tried to schedule a deprecated task, we can choose to either execute it as always with a warning, or reject the scheduling of the new task (presumably, this would be up to the developer).
4. At some point, a deprecated task type could be removed entirely, but the semantic to that depend on how we treat breaking changes going forward (major version? etc.)

The nice part about this approach is that developers could actually provide a "clean up" implementation (instead of the original implementation) when they deprecate the task type - so it could be used to gracefully clean up the legacy task, or unschedule it if it's a recurring task.

[elasticmachine]

Pinging @elastic/kibana-alerting-services (Team:Alerting Services)

[mikecote]

Closing as we've implemented a deprecated task types list in Task Manager that developers can use to remove their task definition from Kibana.