Some time ago, some projects needed the 'Category' column in their issue list. In those days, this could only be done centrally, for all projects. As most projects do not seem to maintain the 'Category' settings, and as Redmine now allows for predefined specialized 'Issue' views (Custom queries), we have removed the 'Category' column but created a public Custom query 'Issues with Category' still containing that column.
You find Custom queries in the right pane of your project's 'Issues' screen. So if you want to see your issues with their respective category, just click on 'Issues with Category' and you'll be fine.
BTW, anyone can create their private Custom queries. Create your favorite listing using the small '> Filters' and '> Options' links right above the usual issue list, and then 'Save' to name this query. It will then be displayed in the right pane together with those public queries.