Yes, 100%
We host a few forums for free for various reasons, on those (with the consent of the admins) you might see some ads. Currently they are for Discourse hosting. You also see these ads here on Meta.
This is an experiment that Discourse is running, the ads are very very limited in scope and fully under our control. They are only shown to anonymous (non-logged in) users.
Another reason you might see ads is that admins can choose to show ads for their own communities benefit, using the Discourse Advertising Plugin (Ads)