I believe this is being blocked by Elgg's current notification system which isn't quite efficient and flexible enough for sites with a lot of users. But there's good news: the whole notification system is being rewritten for Elgg 1.9. Once the community site gets upgraded to 1.9 it will be possible to implement more sophisticated notification features.
Cash has been working hard on the new system. He's written literally thousands or rows of code for it and most of the new features have already been merged to the development version of Elgg 1.9.
Ticket about adding a built-in subscription system to Elgg core already exist: https://github.com/Elgg/Elgg/issues/3871 (it's however still waiting to be implemented).
So don't worry, we'll get the subscription system! There's just some other work that needs to be finished first. :)
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