It was actually ſ+z, which is easier to see if you look at it in Fraktur, and is literally the name of the character. I have long been puzzled by how its romanization became „ss“. In older writing ‘s’ was just the terminal form of ‘ſ’ — if you needed two you just wrote ſſ, same as in English into the early 19th century.

Other ligatures like tz and ch did not survive the jump into Roman letterforms, only ß for some reason.

By the way the now obsolete dual letterforms of s (ſ/s) was inherited from Greek. We’re lucky it was still in use in Leibniz’s time or calculus notation would be more confusing!