In the first episode/chapter of Naruto, Naruto steals the scroll containing secret jutsus and, from it, learns Tajū Kage Bushin no Jutsu, the multiple version of the corporeal alternative to Bushin no Jutsu (which they learn in school). While Kage Bushin no Jutsu seems to take way more chakra and is a lot harder on the user than Bushin no Jutsu, it doesn't seem particularly egregious. Additionally, a decent number of characters seem to use it, if the Wikia article is to be believed. While most characters who use Kage Bushin no Jutsu don't make multiple clones, it doesn't seem like a massive leap from one to the other. So why is it kept secret and sealed away in the Scroll of Seals when other jutsus that are incredibly hard on a person or even potentially fatal (like opening the Eight Gates) aren't?
It’s a forbidden jutsu because of the amount of chakra it requires. Along with that with certain users of the jutsu it could increase their numbers from 1 to 1000 creating a small army.
The eight gate that you gave in the example is a forbidden jutsu and not just everyone can learn it (i.e. sealed). The only known users (manga canon) of this jutsu are Might Guy and Rock Lee, both are Taijutsu masters with the first was even commended as the greatest Taijutsu user ever by none other than Uchiha Madara. Even Lee are not able to open all 8 gates, only Might Guy can. Other than ability, it was also suggested that Might Guy has yet to teach Lee how to open all 8 gates. Lee was limited to the 6th gate. This means that the jutsu is somewhat "sealed" by Guy.
Another forbidden jutsu that was not sealed is the Rasen Shuriken. But that is because Naruto created it himself.
The rest of the kinjutsu are used only by powerful ninjas, sometimes by stealing the scroll (like Deidara did), or the kinjutsu requires Kekkei Genkai (like Izanami and Izanagi).
Thus the conclusion is that other kinjutsu are sealed, just like Kage Bunshin no Jutsu was.
The shadow clone jutsu was invented by Tobirama senju. The reason it is a forbidden jutsu is because it drains the user's chakra. When kakashi was fighting PAIN he lost a decent amount of chakra to the clone.