With the addition of Easy to the leaderboards, I think difficulties in Shinobi should be explained to avoid confusion. You should know that not all versions of Shinobi have all difficulty settings, and not all identically-named difficulties are identical.

First of all, there are two mechanically different variants of Shinobi for the PS2, released in various regions:

• US
• JP/EU/KR

Across both of these variants, there are a total of four available difficulties, listed here in order of enemy health and therefore challenge rating¹:

• Easy
• Normal
• Hard
• Super

The US version has access to Normal, Hard, and Super, whereas the other versions have access to Easy, Normal, and Hard. In order to play the difficulty that's unavailable for your version, you will have to find yourself the respective version of the game, potentially ran on a region-unlocked PS2 or via emulator.

In particular, the US version does not have access to the Easy difficulty, which is the cause for this post. The Normal and Hard difficulties appear to be identical between versions though, making Normal an optimal "main" category for this speedgame.

If you want to go further into the differences, placement of Oboro Clan Coins differs between versions, so don't be surprised when you're looking for a guide and the descriptions don't make sense to you. Because there is no "All Coins" or w/e speedrun at this point, the above list does not differentiate between different coin locations. According to The Cutting Room Floor:

(...) The US version removed Easy, shifted the Normal-level collectables to Hard, and added an extra "Super" difficulty with the old Hard-level collectables (...)

Another functional difference for the EU version of the game specifically is the selection between 50Hz and 60Hz. While 50Hz is currently untested in any form, it seems sensible to me to require EU runs to be done on 60Hz, as that will result in the identical NTSC output that the other versions have (this is because the PAL PS2s output actual NTSC instead of PAL60).

I have no information about the PS3 version of the game, and I can't test it because I don't have access to it.