As we know, shiki fuujin jutsu summons the death god which takes the soul of the summoner and target. But is it a summoning jutsu or just a kind of chakra attack (like rasengan, chidori etc)?
- Performer need to an additional donation of blood on the hand they signed the contract with.
- Mould their chakra with hand seals and then plant the hand they signed the contract with at the location they wish to summon the creature.
We know that when Hiruzen Sarutobi performing this technique, he skipped steps such as donate summoner's blood and plant summoner's hand to the ground, or anywhere. AFAIK, Summoning Jutsu is classified as Ninjutsu and Space-Time Ninjutsu, where Shiki Fūjin is classified as Ninjutsu, Fūinjutsu and Kinjutsu.
As Space-Time Ninjutsu Definition says, all summoning-based techniques are space–time manipulation related, as they warp the targets through a dimensional void to the summoner's location. This makes a clear classification where Shiki Fūjin is not a Summoning Jutsu, because Shiki Fūjin doesn't classified as Space-Time Ninjutsu. I guess even though Hiruzen is indeed summoning Shinigami, it can't easily state that it is a Summoning Technique since Shinigami is type of soul creature, maybe it is a kind of Summoning Kinjutsu instead of Summoning Ninjutsu or some sort of thing.
In a nutshell, Shiki Fūjin is mainly Fūinjutsu, which seal objects, living beings, chakra, along with a wide variety of other things within another object, such as sealing scroll.