You can fetch them from the API, the list is quite short though, and a lot of them are just old radicals.
The current list of hidden items
Ra (radical)
Raptor Cage (radical)
Drawer (radical)
Cyclops (radical)
Roof (radical)
Hercules (radical)
Six (radical)
Grenade (radical)
Task (radical)
悪女 (vocabulary)
里心 (vocabulary)
人性 (vocabulary)
面 (vocabulary)
性愛 (vocabulary)
可分 (vocabulary)
乱交 (vocabulary)
借財 (vocabulary)
文句する (vocabulary)
賀状 (vocabulary)
麦粉 (vocabulary)
聖日 (vocabulary)
損害する (vocabulary)
超音速 (vocabulary)
堀川 (vocabulary)
原潜 (vocabulary)
今日は (vocabulary)
相図 (vocabulary)
大いに (vocabulary)
借款 (vocabulary)
媒酌 (vocabulary)
Worm (radical)
Corn (radical)
Egg (radical)
Trident (radical)