Putting categories in the sidebar results in a lot of clutter on more categorised projects. Also categories are often not things readers would even want to see. See: https://meta.wikimedia.org/wiki/File:Wikipedia_is_scary_joan_of_arc.png
- Limit number in sidebar, select out more likely to be useful ones: T198026: Sort and prioritise categories in the sidebar
- Put full list, including hidden categories, at bottom of page as usual
- Don't put categories in sidebar at all all unless in 3-column view
See also: T175714: Show during page editing the categories on the page instead sidebar