Well, the Reaper Death Seal is a sealing jutsu that originated in Uzushiogakure which was the home of the Uzumaki clan. Which has been wiped out. Kushina had taught this jutsu to Minato, who I'm guessing taught it to the 3rd Hokage. Or it was the previous Jinchuriki before Kushina that did.

Because of this, why would the other lands have this technique since most of the Uzumaki clan is dead?

Also the Naruto Fillers, at least they made sense for the most part...unlike bleach where they're fighting the espada's then you see Ichigo playing soccer for no reason....

I feel like alot of people know about the reaper death seal technique honestly. Not to many but alot

Well, yes. But remember, you give your soul to the Shinigami and DIE after using the Reaper Death Seal.

A lot? The series literally introduced 3 people who knew about who wasn't a part of the Uzumaki Clan (the clan which was already wiped out, bringing their secrets with them): Minato, Hiruzen, and Orochimaru. It's not even certain if Nagato, Karin, or Kabuto knew anything about it. The jutsu also kill the person who uses it, and if the person who use it dies before the jutsu is completed, it won't take full effect. It's literally suicide bombing to kill 1 target with a decent chance of failure. And both people sealed can't usually be revived since their souls are consummed by the reaper, with the only loophole being known by Orochimaru who's obsessed with cheating death and spent his entire life learning of jutsus and gathering information no one else would know. Basically discounted Ra's Al ghul and discounted Batman rolled into one. To even execute said loophole requires the person to slice him/herself opened, which would usually kill anyone who isn't Orochimaru, who conveniently possessed/soul transfered to a white Zetsu who was spying on them. So really, if you think about it, every step of this jutsu dooms the person or people who use it to eternal hell for most people, so unless you have no values for your own life and other people, sociopath much, you would not usually consider using this skill, and as stated above, the jutsu itself is a kept-secret, no one else aside from the mentioned people even knew about the jutsu.

Also, you understood the whole plot of Naruto wrong. It was never about killing/ destroying the tailed beasts, it was about villages and lands having control over tailed beasts to have a competitive edges vs one another. So why would 9 people even consider removing their edge and make their own land weaker compare to other lands?

Even Minato decided not to seal everything, but only half in himself and half in his son believing both Naruto and Konoha will be needing Kyuubi. Or did you just skip that whole speech about how it's the best solution he could come up with that will be best for everyone right before he died? And no, he didn't just do that because he was sacrificing for Konoha, he had to seal half of Kurama in himself in fear that 100% of Kurama would overwhelm Naruto and turn his son towards darkness. And what happened? 50% of Kurama still almost did just that, multiple times.

So, um, are you half-asleep while reading/watching Naruto or something? Because these are basic information.