Syntactically, this の is a noun, and everything before it is a relative clause. But I think it's best to treat this の as "some special noun working as a placeholder". It's also different from what we usually call a nominalizer (which turns 見る "see" to 見るの "seeing", etc.).
Your sentence is a cleft sentence, a special construction to focus a certain part of a sentence. This の roughly corresponds to the "placeholder it" in English cleft sentences like "It's John who ate the cake". You can see that this it is specially referring to a person, which is odd outside of this construction. Likewise, の in a cleft sentence is a special noun which is used in place of many things including 者/人, 物, 場所/所 and 時/時間, and it is unlikely to be an abbreviation of any particular word.
Who was it that came here yesterday?
(This can be rephrased as "Who was the person who came here yesterday?", but that does not mean this it is short for person.)
When was it that he came here?
Where was it that he came yesterday?
Why was it that he came here yesterday?
(For this one, I don't know how to rewrite this using a normal noun and a normal relative clause)