I don't think the Flarum Team will implement it, there's currently no plan to change how the core threads work, nor will we add new core extensions like that before multiple stable releases.
And no third-party extension developer has picked up on that yet. By my estimate it's way too much work for a free/OSS extension. But I'm sure you can get the development commissioned if you need it.
Even though we will probably never implement this as a core extension, we'd love to tackle any extensibility issue that might prevent the development of such an extension by a third party. So if anyone is working on it, please submit tickets for the eventual extensibility difficulties.