On the internet, nobody knows you’re a raccoon

I also live in Europe and where I live it’s not common

It’s an identity matrix. You multiple a vector with it and the result is still the same (identical) vector

I think they used whole sentences back in the days and it was in the printing press era that they started abbreviating things to letters and symbols

That’s one theory why x is used in math so much: in times of the printing press, the letter x was barely used so always free