From Plex Support Article:
Plex allows you to optionally sync your watched/played state and personal ratings of movies and episodes with your Plex account online. That means that if you watch a movie on a friend’s server, it can update your account so that the movie will also be marked as watched on Plex’s Discover feature, our free Movies & Shows streaming service, and any Plex Media Server you have access to.
Does this mean I can do away with PlexTraktSync already?
From what I gather, if you have multiple Plex server, you can sync your watch state across all of them. Also, if you decide to migrate an existing Plex server or re-start a new server from scratch, it won’t be much of a pain carrying over your watch states, for you, the server owner, and your users. That’s neat. No more plugin, no more Trakt, and your users whom you’ve shared your library with can enable the same to their account (this settings is account-specific).
I’ve enabled this feature in my Account Settings. Now, let’s wait for it to do its magic.