Ich habe nichts gewußt.

I thought that this was spelled “gewusst”…and the “u” is short…is this an older spelling, pre-spelling reform?

To me it seems correct, it is what I learned in school, but the form with the “ss” would technically not be incorrect either.

According to Wikipedia apparently they did stop using the ß in Swiss Standard German
ß - Wikipedia last century, but in Standard German it is still used.