I've noticed that if I change the number after "Category" in a subforum's URL, I can find hidden subforums:
https://powdertoy.co.uk/Discussions/Categories/Topics.html?Category=7 (inaccessible)
https://powdertoy.co.uk/Discussions/Categories/Topics.html?Category=10 (inaccessible, Web Archive shows it to be an element suggestion subforum)
https://powdertoy.co.uk/Discussions/Categories/Topics.html?Category=13 (Seems to be another element suggestion subforum, closed/locked around 2013)
https://powdertoy.co.uk/Discussions/Categories/Topics.html?Category=14 (inaccessible)
What are the other 2 inaccessible ones? Some sort of mod/admin sections?
This is going to be a big nerco, but I think they were pages before the website changed because it says "access denied" and it was also a part of April fools, probably a format change/site locks because of the improved editors and "hidden" shenanigans, but you gotta get some fun in your forums, right?
Mid 2012, if memory serves me correctly. When the site was redesigned.