Lurker Mercury does offer a basic notification system for available extension updates, but that does not extend to
flarum/core itself. Covering core updates is quite possible to add however.
What it does not offer, is a channel for the core team to be able to push any text or convey any sort of urgency along with it, it simply checks packagist/extiverse every so often to see if a new release is available.
AFAIK there are no concrete plans yet, but I do know we are considering retiring the mercury extension in it's current form and merging some of it's functionality into the upcoming package manager.