The idea is of course similar to modern day spoilers but the concept proposal was quite old. Moreover, the details were revealed inside the text not separately, i.e. simply added further sentences to the text. The name was something like "expandable hypertext"
Any hints would be appreciated!
Progressive Disclosure. This design pattern intends to reduce UI clutter, confusion and cognitive workload so that the user’s focus remains on the interface.
The example you described is achieved one way by using HTML5 tags summary and details.
I found it: Stretchtext. The concept was invented by Ted Nelson in 1967.
