It's pretty obvious that the 3 hokages knew of the technique. I don't think Madara created the jutsu, his pride in his mangekyou sharingan was too great for him to be relying on others for power. Most likely Rikudou Sennin or the 1st Created it. But what I'm thinking is that it was a secret technique, along with other death-related jutsus, of the Senju Clan, reason why they were so powerful, enough to rival the EMS and his uchihas.

The Mokuton bloodline of the senju clan had the power to create life, it would make sense if they had also successfully learned to control the power of death, giving them godly powers. The first himself could've used the technique to overpower and defeat madara who wouldn't really have a counter to it. Then after defeating madara, when peace was coming to konoha, he declared the death jutsus forbidden, only to be studied by the hokages in case to use them as a last resort to protect the village. That's why the 4th used that knowledge and created shiki-fujin.

But Oro's ambitions led him to discover and study those jutsus. It would also mean that Tsunade has acquired that knowledge and being the granddaughter of the 1st, she should be able to master the kinjutsus very well. I think that Shiki Fujin balances Edo Tensei in that souls brought back from the underworld will be caught within the belly of the deathgod, but only a person strong enough would be able to summon them back from the shinigami's belly as Oro was trying to do with Fourth Hokage.

It would be cool if the 1st had actually used Edo Tensei to summon the Rikudou Sennin and help him fight against Madara, and then released his soul with some sort of life-giving jutsu until the sage was reincarnated as Pain aka nagato years later during a time of massive war and suffering. And now, in a twist of fate, Madara's using the rinnegan power for his own benefit.