Nothing. Nada. No 3rd party cookies or 3rd party XHR, nothing.
It’s small, but it’s just one of those little signs that Purism is still pure. Kudos to them for proving that it’s totally possible to run and maintain a forum and slick website without all of the garbage seen ubiquitously across the interwebs.
Which web site exactly?
forums.puri.sm? The latter is using Discourse. There are other Purism web sites that might use different technologies again e.g.
source.puri.sm (the latter having the Community Wiki on it as well as functionality for cloning, merging and issue tracking).