^Sertorius is correct. If this changed at all, which I don't believe it did, it was an intentional and logical change. Blame the original devs for putting a straight sword in the same proficiency slot as the scimitar just becuase it was 'oriental-seeming'.
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If you read the description of a non-magical ninja-to, it says that it has a straight blade, so IMO the long sword model would make sense.
So it should use katana model, not long sword or scimitar.
Looks like a straight blade to me. Still closer to a long sword than a katana.
Although they were later more popular straighter, originally are Ninja-to's lightly curved.
( http://www.kotokatana.com/john-lee-ninja-swords-togakure-ryu-ninjato.html )