The official announcement was mentioned in Matrix’s blog post:
Next release 0.4.2 should also be published on F-Droid with push support using UnifiedPush.
In other news, thanks to Joshua, NeoChat now supports push notifications based on UnifiedPush.
Interestingly, the Element X issue regarding implementing UnifiedPush, has not been updated, and there is no related merge request that I can find. NeoChat appears to have their changes buried in merge requests.
Technologically, there’s a little more to it than only that, but, in practice, that’s essentially what it does.
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UnifiedPush, itself, is just the standard; services like Ntfy then implement it.