Guild Wars 2

Looking at the screenshot – There are whole pages of completed story journal items that are just cluttering up what I need to do next.

There is no right-click hide/remove options but I’ve seen others do it in the past. How do I remove completed quests/milestones and the like?

I don’t think you can hide individual story steps. The best you can do is collapse the whole category (for example, you can collapse Living Story Season 2) using the arrows to the left of each section.