Under "Page previews content/Text", the page for the Popups extension says:
For third parties we encourage using the Page Content Service to enjoy using Popups with your local wiki. You can also use the Extension:TextExtracts extension. This extension has various caveats and we do not actively support use of this API.
I came across this when I deployed the Popups extension to Icannwiki earlier tonight and saw that, while some popups worked, most I tried did not. To see this, compare "ASCII" (working) and "DNS" (fails) under Internationalized Domain Name. I did some digging, and, not finding any obvious errors on the server, I went to look at the documentation. That's when I found the above quote and then the (seemingly) contradictory statements from @Jdforrester (WMF) below.
If we are really meant to not use the TextExtracts extension, it would help to outline how to set up PCS. Alternatively, maybe there is a list of tasks in phabricator that we can attack.