FYI, it doesn’t work particularly well. All this does is hide the individual posts of the ignored individual within discussion threads. You will still see threads started by this person in forum views, and (in what strikes me as an especially egregious oversight) the “other recommended topics” list at the bottom of a thread page will still be polluted with items started by the ignored individual.
It goes to the usage bias of the Discourse designers, who disagree with the inclusion of an ignore feature on principle. If someone is being stupid, they say, you should be able to argue them to your position, and barring that you can just skip over their contributions; actually suppressing their content is antithetical to healthy conversation. Of course, everyone else on the internet vehemently disagrees, so the ignore feature was added with reluctance, but it’s limited and underfeatured.