I'm using Elatic Stack 6.4 (rolled upgraded from 6.3.2).
I've noticed that, I'm no longer getting any email alerts (removed/added elasticsearch node) in 6.4 as I did in 6.3.2 (no other changes were done, besides rolling upgrade from 6.3.2 to 6.4).
The email action is still present - the features documented as "X-Pack" in versions 6.2 and below are now documented under "Elastic Stack". You can find the 6.3/6.4 documentation for the particular feature you're looking for at Email Action | Elastic Stack Overview [6.4].
I'm not aware of any changes in 6.4 that would prevent emails from firing as they were before - this would be difficult to troubleshoot without more information.
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