There's very little context here for MediaWiki users coming from 1.27 LTS. The linked Page Content Service Project Google Doc at https://docs.google.com/document/d/15EAaDwK70i2pT4Xkj2Kdp9Q8XsYKGUQkJTSj5Rq8xiE/edit is not public. There's no information on this page about whether this is built into MediaWiki, an extension, or an external service, and if an extension or external service where to find it or how to install or configure it.
Topic on Talk:Page Content Service/Flow
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It's a service designed and built for Wikimedia. How did you come to read it, and what problem were you hoping it might solve? Maybe there's an alternative to which I could point you.